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Work And Energy - Class 9 Science (Compulsory Test) - Class 9 MCQ


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30 Questions MCQ Test - Work And Energy - Class 9 Science (Compulsory Test)

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Work And Energy - Class 9 Science (Compulsory Test) - Question 1

When a light and a heavy body have equal K.E, then which one has a greater momentum?

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 1
Heavy body have more momentum. bcz momentum = mass.velocity .so the mass of the heavy body is more than the light body.
Work And Energy - Class 9 Science (Compulsory Test) - Question 2

In SI system, the unit of P.E. is:

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 2
The unit of P.E. in SI system is J (Joules).
Explanation:
- SI system: The International System of Units (SI) is the modern form of the metric system and is the most widely used system of measurement. It is used in science, engineering, and everyday life.
- P.E.: P.E. stands for Potential Energy, which is the energy possessed by an object due to its position or condition.
- Unit of P.E.: In the SI system, the unit of energy is Joule (J). It is named after James Prescott Joule, an English physicist who contributed to the study of energy and thermodynamics.
- Joule: Joule is the derived unit of energy in the SI system. It is defined as the energy transferred to an object when a force of one newton is applied over a distance of one meter.
- Other options:
- Erg: Erg is a unit of energy in the CGS (centimeter-gram-second) system. 1 erg is equal to 0.0000001 joules. It is not commonly used in the SI system.
- Dyne-cm: Dyne-centimeter is a unit of work or energy in the CGS system. 1 dyne-centimeter is equal to 0.0000001 joules. It is not commonly used in the SI system.
- None of these: This option is incorrect as the correct unit of P.E. in the SI system is Joule (J).
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Work And Energy - Class 9 Science (Compulsory Test) - Question 3

Kilowatt hour (kWh) represents the unit of:

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 3
Kilowatt hour (kWh) represents the unit of energy.
Explanation:
The kilowatt hour (kWh) is a unit of energy commonly used to measure electricity consumption. Here's a detailed explanation:
Definition:
- The kilowatt hour (kWh) is a unit of energy equivalent to the consumption of one kilowatt (1 kW) of power over a period of one hour.
- It is widely used by utility companies to measure and bill for the amount of electricity consumed by households, businesses, and industries.
Understanding the answer:
- The term "kilowatt" represents power, which is the rate at which energy is consumed or produced.
- The term "hour" represents a unit of time.
- The combination of "kilowatt" and "hour" signifies the amount of energy consumed or produced over a specific period.
Key points:
- The kilowatt hour (kWh) is used to measure the amount of energy consumed or produced.
- It is a common unit of measurement for electricity usage.
- It is not related to momentum or any other physical quantity.
- The kilowatt hour is calculated by multiplying the power in kilowatts (kW) by the time in hours.
Example:
- If a device has a power rating of 1 kilowatt (1 kW) and it is operated for 2 hours, the energy consumed would be 2 kilowatt hours (2 kWh).
- Similarly, if a household consumes 500 kilowatt hours (500 kWh) of electricity in a month, it means that they have used 500 units of energy during that period.
In conclusion, the kilowatt hour (kWh) represents the unit of energy, specifically the amount of energy consumed or produced over a specific period of time.
Work And Energy - Class 9 Science (Compulsory Test) - Question 4

When speed of a motor car increases six times, then kinetic energy increases by

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 4

To find the increase in kinetic energy, we need to understand the relationship between kinetic energy and speed.
The kinetic energy (KE) of an object is given by the equation:
KE = (1/2) * m * v^2
where m is the mass of the object and v is its velocity.
Let's assume the initial speed of the motor car is v1 and its kinetic energy is KE1. When the speed increases six times, the new speed becomes 6v1 and the new kinetic energy becomes KE2.
To find the relationship between KE1 and KE2, we can compare the two equations:
KE1 = (1/2) * m * v1^2
KE2 = (1/2) * m * (6v1)^2 = (1/2) * m * 36v1^2
Now, let's calculate the ratio of KE2 to KE1:
KE2/KE1 = ((1/2) * m * 36v1^2) / ((1/2) * m * v1^2)
= 36v1^2 / v1^2
= 36
Therefore, the kinetic energy increases by a factor of 36, which means it increases 36 times. So, the correct answer is B. 36 times.
Work And Energy - Class 9 Science (Compulsory Test) - Question 5

1 kWh equals:

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 5

To determine the correct answer, we need to convert 1 kWh to Joules.
1 kilowatt-hour (kWh) is equal to the energy consumed or produced by a 1-kilowatt (1,000 watt) device for 1 hour.
To convert from kilowatt-hours to Joules, we can use the following conversion factors:
1 kilowatt-hour (kWh) = 3.6 × 10^6 Joules
Now, let's check each option:
A: 36 × 10^2 Joules
This is equal to 3600 Joules, which is not the correct conversion.
B: 36 × 10^4 Joules
This is equal to 360,000 Joules, which is not the correct conversion.
C: 3.6 × 10^6 Joules
This is the correct conversion. 1 kWh is equal to 3.6 × 10^6 Joules.
D: None of these
This is not the correct answer because option C is the correct conversion.
Therefore, the correct answer is C: 3.6 × 10^6 Joules.
Work And Energy - Class 9 Science (Compulsory Test) - Question 6

When speed of the moving object is doubled its

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 6
When the speed of a moving object is doubled, several factors are affected. Let's break down the options and explain each one in detail:

  1. Acceleration is doubled:

  2. Acceleration is the rate at which an object changes its velocity. When the speed of a moving object is doubled, its acceleration does not necessarily double. Acceleration depends on the force acting on the object, not just its speed. So this statement is not correct.


  3. Momentum becomes four times more:

  4. Momentum is defined as the product of an object's mass and its velocity. When the speed of a moving object is doubled, its momentum also doubles. This is because momentum is directly proportional to velocity. Therefore, this statement is not correct.


  5. Kinetic energy is increased to four times:

  6. Kinetic energy is defined as the energy an object possesses due to its motion. It is directly proportional to the square of the object's velocity. When the speed of a moving object is doubled, its kinetic energy is increased to four times. This is because the kinetic energy formula includes the square of the velocity. Therefore, this statement is correct.


  7. Potential energy is increased:

  8. The potential energy of an object depends on its position in a gravitational field. It is not directly related to the speed of the object. So when the speed of a moving object is doubled, its potential energy is not necessarily increased. Therefore, this statement is not correct.



In conclusion, when the speed of a moving object is doubled, its kinetic energy (option C) is increased to four times.
Work And Energy - Class 9 Science (Compulsory Test) - Question 7

When time taken to complete a given amount of work increases, then

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 7
Explanation:
When the time taken to complete a given amount of work increases, it means that the work is being done at a slower rate. This has an impact on both power and energy. Let's break it down:
A. Power:
Power is defined as the rate at which work is done or energy is transferred. It is calculated by dividing the amount of work done by the time taken to do the work. In this scenario, since the time taken to complete the work has increased, the power decreases. This is because the work is being done at a slower rate.
B. Energy:
Energy is the capacity to do work. It is the product of power and time. In this scenario, since the time taken to complete the work has increased, the energy also increases. This is because more time is being taken to do the same amount of work, resulting in more energy being expended.
Therefore, the correct answer is B. Power decreases.
Work And Energy - Class 9 Science (Compulsory Test) - Question 8

When the force applied and the displacement of the body are inclined at 90° with each other, then work done is

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 8

When the force applied and the displacement of the body are inclined at 90° with each other, the work done is zero.


Work done is the product of the force applied on an object and the displacement of the object in the direction of the force. Mathematically, work done (W) is given by:


W = F * d * cosθ


Where:
F is the magnitude of the force applied,
d is the magnitude of the displacement of the body, and
θ is the angle between the force and the displacement.


In this case, the angle between the force and the displacement is 90°. When the angle is 90°, the cosine of the angle becomes zero.


So, W = F * d * cos90° = F * d * 0 = 0


Therefore, the work done is zero when the force applied and the displacement of the body are inclined at 90° with each other. Hence, the answer is option C: zero.

Work And Energy - Class 9 Science (Compulsory Test) - Question 9

The KE. of a body in increased most by doubling its

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 9

To determine which factor will increase the kinetic energy (KE) of a body the most, we can analyze the equation for kinetic energy:
KE = (1/2)mv^2
Where:
- KE is the kinetic energy
- m is the mass of the body
- v is the velocity (speed) of the body
From the equation, we can see that the kinetic energy depends on both the mass and the speed of the body.
Analysis:
To determine which factor will have the greatest impact on the kinetic energy, we can consider the following:
1. Mass: If the mass of the body is doubled, the kinetic energy will also double. This is because the mass is directly proportional to the kinetic energy in the equation.
2. Speed: If the speed of the body is doubled, the kinetic energy will increase by a factor of four. This is because the speed is squared in the equation. Thus, a change in speed has a greater impact on the kinetic energy compared to a change in mass.
Conclusion:
Based on the analysis, we can conclude that the kinetic energy of a body is increased the most by doubling its speed. Therefore, the correct answer is option C: speed.
Work And Energy - Class 9 Science (Compulsory Test) - Question 10

If a force F is applied on a body and it move with velocity v, then power will be :-

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 10

Power = Work/ Time.
Work = Force x displacement
So, now Power = Force x displacement / Time.
But displacement / Time = velocity
According to the question, Force is F and V is velocity so,
Power = F x v

But power is a scalar quantity so, we have dot product for power so......
Power = Fv or F.v

Work And Energy - Class 9 Science (Compulsory Test) - Question 11

Work done by a centripetal force

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 11
Work done by a centripetal force
The work done by a force is defined as the product of the magnitude of the force and the distance over which the force is applied. In the case of a centripetal force, which is directed towards the center of a circular path, the work done can be analyzed as follows:
1. Definition of work: Work is defined as the product of the force and the displacement of the object in the direction of the force. Mathematically, work (W) is given by the equation W = F * d * cos(theta), where F is the force, d is the displacement, and theta is the angle between the force and the displacement vectors.
2. Centripetal force: A centripetal force is a force that acts towards the center of a circular path and is responsible for keeping an object moving in a curved path. It is always perpendicular to the displacement of the object.
3. Perpendicular displacement: The displacement of an object moving in a circular path is always perpendicular to the direction of the centripetal force. This means that the angle between the force vector and the displacement vector is 90 degrees (cos(90) = 0).
4. Work done by a centripetal force: Since the angle between the force and the displacement vectors is 90 degrees, the cosine of 90 degrees is zero. Therefore, the work done by a centripetal force is always zero.
Therefore, the correct answer is D: The work done by a centripetal force is always zero.
Work And Energy - Class 9 Science (Compulsory Test) - Question 12

A 1 kg mass falls from a height of 10 m into a sand box. What is the speed of the mass just before hitting the sand box? If it travels a distance of 2 cm into the sand before coming to rest, what is the average retarding force?

 

(Please note that Value of g = 9.8 m/s2)

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 12
Problem: A 1 kg mass falls from a height of 10 m into a sand box. What is the speed of the mass just before hitting the sand box? If it travels a distance of 2 cm into the sand before coming to rest, what is the average retarding force?

Step 1: Finding the initial velocity
Given:
- Mass (m) = 1 kg
- Height (h) = 10 m
- Acceleration due to gravity (g) = 9.8 m/s^2
We can use the equation of motion, which relates the final velocity (v), initial velocity (u), acceleration (a), and displacement (s):
v^2 = u^2 + 2as
Since the mass is falling freely, the initial velocity (u) is 0.
Therefore, the equation becomes:
v^2 = 0 + 2gh
Substituting the given values:
v^2 = 2 * 9.8 * 10
v^2 = 196
v = √196
v = 14 m/s
Therefore, the speed of the mass just before hitting the sand box is 14 m/s.
Step 2: Finding the average retarding force
Given:
- Displacement (s) = 2 cm = 0.02 m
- Mass (m) = 1 kg
- Initial velocity (u) = 14 m/s
- Final velocity (v) = 0 m/s
We can use the equation of motion, which relates the final velocity (v), initial velocity (u), acceleration (a), and displacement (s):
v^2 = u^2 + 2as
Solving for acceleration (a):
a = (v^2 - u^2) / (2s)
a = (0^2 - 14^2) / (2 * 0.02)
a = (-196) / (0.04)
a = -4900 m/s^2
The negative sign indicates that the acceleration is in the opposite direction of motion, i.e., retarding force.
To find the average retarding force (F), we can use Newton's second law of motion:
F = ma
F = 1 * (-4900)
F = -4900 N (Note: The negative sign indicates that the force is in the opposite direction of motion)
Therefore, the average retarding force is 4900 N.
Answer: The speed of the mass just before hitting the sand box is 14 m/s, and the average retarding force is 4900 N.
Work And Energy - Class 9 Science (Compulsory Test) - Question 13

Work done in moving a 50 kg block through a horizontal distance of 10 m by applying a force of 100N which makes an angle of 90° with the horizontal is

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 13
Given:
- Mass of the block (m) = 50 kg
- Distance moved (d) = 10 m
- Applied force (F) = 100 N
- Angle between the force and the horizontal (θ) = 90°
Work done formula:
The work done (W) is given by the formula:
W = F × d × cos(θ)

Substituting the given values into the formula:
W = 100 N × 10 m × cos(90°)
Since cos(90°) = 0, the work done is:
W = 100 N × 10 m × 0
W = 0 Joule
Therefore, the work done in moving the block is 0 Joule.
Work And Energy - Class 9 Science (Compulsory Test) - Question 14

kWh is the unit of

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 14
Unit of kWh
- The unit kWh stands for kilowatt-hour.
- It is a unit of energy.
- Energy is the capacity to do work or the ability to cause change.
- The kilowatt-hour is commonly used to measure the amount of electrical energy consumed or produced over time.
- It is a derived unit, which is a combination of two other units: kilowatts (kW) and hours (h).
- 1 kilowatt-hour is equal to 1 kilowatt of power used or generated over a period of 1 hour.
- The kilowatt-hour is used in utility bills to measure electricity usage in homes and businesses.
- It is also used to quantify the energy output of appliances, such as refrigerators, air conditioners, and light bulbs.
- The kilowatt-hour is a practical unit for measuring energy because it relates power (kW) to time (hours), which are both essential factors in energy consumption or production.
Work And Energy - Class 9 Science (Compulsory Test) - Question 15

An elevator is designed to lift a load of 1000 kg through 6 floors of a building averaging 3·5 m per floor in
6 sec. Power of the elevator, neglecting other losses, will be

 

(Please note that Value of g = 9.8m/s2)

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 15
Power =work/time =force ×displacement/time = mass×acceleration due to gravity× displacement /time = 1000×9.8×3.5×6/6= 3.43× 10*4 watt.
Work And Energy - Class 9 Science (Compulsory Test) - Question 16

The work done by a body is directly proportional to :

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 16
The work done by a body is directly proportional to:
Force acting on the body:
- The work done by a body is directly proportional to the force acting on the body.
- When a force is applied to a body and it undergoes displacement, work is done on the body.
- The greater the force acting on the body, the more work is done.
Displacement produced in the body:
- The work done by a body is also directly proportional to the displacement produced in the body.
- Displacement refers to the change in position of the body.
- When a body undergoes displacement, work is done on the body.
- The greater the displacement produced in the body, the more work is done.
Both (A) and (B):
- The work done by a body is directly proportional to both the force acting on the body and the displacement produced in the body.
- This means that increasing either the force or the displacement will result in an increase in the work done by the body.
- Mathematically, work done (W) is given by the equation W = F * d, where F is the force and d is the displacement.
- Therefore, both the force and displacement play a crucial role in determining the work done by a body.
In conclusion, the work done by a body is directly proportional to both the force acting on the body and the displacement produced in the body. Increasing either the force or the displacement will result in an increase in the work done by the body.
Work And Energy - Class 9 Science (Compulsory Test) - Question 17

Work done is said to be positive when a force causes displacement:

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 17
Explanation:
When a force causes displacement, the work done can be positive, negative, or zero. In this case, we are considering the scenario where the work done is positive. Let's break down the options and analyze them:
A: In its own direction:
- This means that the force and displacement are in the same direction.
- When a force acts in the same direction as the displacement, the work done is positive.
- For example, if you push a box forward and it moves forward, the force and displacement are in the same direction, resulting in positive work done.
B: In the direction opposite to the applied force:
- This means that the force and displacement are in opposite directions.
- When a force acts in the opposite direction to the displacement, the work done is negative.
- For example, if you push a box forward, but it moves backward, the force and displacement are in opposite directions, resulting in negative work done.
C: In the direction at right angles to the direction of the applied force:
- This means that the force and displacement are perpendicular to each other.
- When a force acts at a right angle to the displacement, the work done is zero.
- For example, if you push a box sideways, but it moves forward, the force and displacement are perpendicular, resulting in zero work done.
D: None of the above:
- This option is incorrect because one of the options (option A) is correct.
Therefore, the correct answer is A: in its own direction. When a force causes displacement in its own direction, the work done is positive.
Work And Energy - Class 9 Science (Compulsory Test) - Question 18

How much time will be required to perform 520 J of work at the rate of 20 W?        

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 18
Given:
- Work = 520 J
- Power = 20 W
To find:
- Time required to perform the work
Formula:
- Power = Work / Time

- Rearranging the formula: Time = Work / Power
- Substituting the given values: Time = 520 J / 20 W
- Calculating the value: Time = 26 s
Answer:
The time required to perform 520 J of work at the rate of 20 W is 26 seconds.
Work And Energy - Class 9 Science (Compulsory Test) - Question 19

In which case work is not done:

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 19
Case where work is not done:
Answer: C - a standing man holding a suitcase in his hand
Explanation:
Definition of work:
Work is defined as the transfer of energy that occurs when a force is applied to an object and the object is displaced in the direction of the force.
Conditions for work to be done:
In order for work to be done, the following conditions must be met:
1. There must be a force applied to the object.
2. The object must move in the direction of the applied force.
Analysis of the given cases:
A: A girl swimming in a pond
- The girl exerts a force against the water by swimming.
- The girl moves through the water in the direction of the force.
- Work is done in this case.
B: A windmill lifting water from a well
- The windmill exerts a force on the water to lift it.
- The water moves in the direction of the force.
- Work is done in this case.
C: A standing man holding a suitcase in his hand
- The man exerts a force against the suitcase.
- The suitcase does not move in the direction of the force.
- No work is done in this case.
D: A sailboat moving in the direction of the wind
- The wind exerts a force on the sail.
- The sailboat moves in the direction of the force.
- Work is done in this case.
Conclusion:
Work is not done when a force is applied to an object, but the object does not move in the direction of the force. In the given cases, the man holding the suitcase is not displacing the suitcase, so no work is done in that case.
Work And Energy - Class 9 Science (Compulsory Test) - Question 20

In which case work is done:

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 20
Work Done in Different Cases:
A: Green plant carrying out photosynthesis
- Photosynthesis is a biological process where green plants convert sunlight, water, and carbon dioxide into glucose and oxygen.
- The process of photosynthesis involves the absorption of light energy, conversion of energy into chemical energy, and synthesis of glucose.
- The plant is performing work in this case as it is converting energy from the sun into chemical energy in the form of glucose.
B: Porter standing at a place and carrying a heavy load on his head
- In this case, the porter is exerting force on the load by carrying it on his head.
- However, since the porter is standing in one place and not moving, no work is being done.
- Work is defined as the product of force and displacement, and since there is no displacement, no work is done.
C: Drying of food grains in the sun
- Drying of food grains in the sun involves the evaporation of moisture from the grains due to the heat from the sun.
- The sun provides the necessary energy to convert the water into vapor, thereby drying the grains.
- This process does involve work as the energy from the sun is being utilized to cause a change in the state of the grains.
D: Trolley rolling down a slope
- When a trolley rolls down a slope, the force of gravity acts on it, causing it to accelerate and move.
- The force of gravity is doing work on the trolley as it is causing a displacement of the trolley.
- The energy from the gravitational force is being transferred to the trolley, and work is being done in this case.
Conclusion:
- In the given cases, work is done in the situations where there is a displacement caused by a force.
- Work is not done when there is no displacement or when the force and displacement are perpendicular to each other.
- Therefore, in this scenario, the correct answer is D: a trolley rolling down a slope.
Work And Energy - Class 9 Science (Compulsory Test) - Question 21

A stone is tied to a string and whirled in a circular path. The work done by the stone is :

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 21
Explanation:
When an object moves in a circular path, the force acting on it is always directed towards the center of the circle. This force is called centripetal force.
Work done by a force is given by the formula:
Work = Force x Displacement x cos(theta)
In this case, the force acting on the stone is always directed towards the center of the circle, while the displacement of the stone is perpendicular to the force. Therefore, the angle theta between the force and displacement is 90 degrees.
When cos(theta) is 90 degrees, the work done is:
Work = Force x Displacement x cos(90) = 0
Hence, the work done by the stone is zero.
Therefore, the correct answer is B: zero.
Work And Energy - Class 9 Science (Compulsory Test) - Question 22

A man carries a suitcase in his hand climbs up the stairs. The work done by the man is :

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 22

To determine the work done by the man while carrying a suitcase up the stairs, we need to understand the concept of work and its direction.
Work:
Work is defined as the amount of energy transferred by a force acting on an object as it moves through a distance. Mathematically, work is calculated as the product of force and displacement, where the force and displacement are in the same direction.
In this scenario, the man is applying a force on the suitcase to lift it up the stairs. Let's analyze the possible directions of force and displacement:
Force:
The force applied by the man is directed upward, opposite to the force of gravity acting on the suitcase. This force is necessary to counteract the gravitational force and lift the suitcase.
Displacement:
The displacement of the suitcase is also directed upward, as the man is moving the suitcase vertically up the stairs.
Since the force and displacement are in the same direction, the work done by the man is positive. The man is exerting energy to lift the suitcase against gravity.
Therefore, the correct answer is Option A: positive.
Work And Energy - Class 9 Science (Compulsory Test) - Question 23

A rocket rises up in the air due to the force generated by the fuel. The work done by the:

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 23
Explanation:
To understand why the answer is option B, let's break down the situation:
1. The rocket rises up in the air due to the force generated by the fuel. This force is generated by the burning of the fuel and the expulsion of the exhaust gases at high speeds.
2. Work is defined as the transfer of energy, and it is calculated by the product of force and displacement. In this case, the work done by the fuel can be considered as the work done by the rocket engine.
3. The rocket engine exerts a force in the upward direction, pushing the rocket against the force of gravity. As a result, the rocket moves upward, and the force of gravity does negative work on the rocket.
4. The work done by the fuel is positive because it is in the same direction as the displacement of the rocket. The force of gravity does negative work because it is in the opposite direction to the displacement of the rocket.
5. Therefore, the correct answer is option B: the fuel does positive work, and the force of gravity does negative work.
Summary:
- The rocket rises up due to the force generated by the fuel.
- Work done by the fuel is positive as it is in the same direction as the displacement of the rocket.
- Work done by the force of gravity is negative as it is in the opposite direction to the displacement of the rocket.
Work And Energy - Class 9 Science (Compulsory Test) - Question 24

One joule work is said to be done when a force of one newton acts through a distance of:

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 24

To determine the distance over which a force of one newton acts to do one joule of work, we can use the formula:
Work = Force × Distance
Given that the work is one joule and the force is one newton, we can rearrange the formula to solve for the distance:
Distance = Work / Force
Substituting the given values:
Distance = 1 joule / 1 newton
Simplifying the expression:
Distance = 1 meter
Therefore, the distance over which a force of one newton acts to do one joule of work is 1 meter. Hence, option C is the correct answer.
Work And Energy - Class 9 Science (Compulsory Test) - Question 25

Work is the product of time and:

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 25
Work is the product of time and power:
Explanation:
- Work is defined as the amount of energy transferred by a force acting through a distance.
- It is calculated by multiplying the force applied to an object by the distance the object moves in the direction of the force.
- However, work also depends on the time it takes to perform the task and the power used.
- Power is the rate at which work is done or the amount of work done per unit time.
- Therefore, work can be expressed as the product of time and power.
- This can be mathematically represented as: Work = Power x Time.
Conclusion:
- From the given options, the correct answer is A: power.
- Work is the product of time and power, as power determines the rate at which work is done.
- So, the statement "Work is the product of time and power" is true.
Work And Energy - Class 9 Science (Compulsory Test) - Question 26

In angle in between the direction of applied force and displacement, for maximum work should be :

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 26

To maximize work, the angle between the direction of applied force and displacement should be zero. This means that the applied force should be in the same direction as the displacement.
Here's a detailed explanation:
1. Understanding Work:
- Work is defined as the product of the magnitude of the applied force and the displacement of the object in the direction of the force.
- Mathematically, work (W) is given by the equation: W = F * d * cosθ, where F is the applied force, d is the displacement, and θ is the angle between the force and displacement vectors.
2. Maximum Work:
- To maximize work, we need to maximize the value of cosθ in the equation.
- The maximum value of cosθ is 1, which occurs when θ = 0° (cos0° = 1).
- This means that the angle between the direction of applied force and displacement should be zero.
3. Illustration:
To understand this concept better, let's consider an example:
- Suppose you are pushing a box horizontally across the floor with a force of 100 N.
- The displacement of the box is also in the horizontal direction, let's say 5 meters.
- If you push the box in the same direction as its displacement, the angle between the force and displacement vectors is zero.
- In this case, the work done on the box will be maximum, given by: W = 100 N * 5 m * cos0° = 500 J.
4. Other Angle Options:
- If the angle between the force and displacement vectors is 90° (option A), the value of cosθ will be zero, and no work will be done.
- If the angle is 45° (option B), the value of cosθ will be less than 1, resulting in less work compared to the angle of zero.
- If the angle is 30° (option D), the value of cosθ will be less than 1, resulting in less work compared to the angle of zero.
Therefore, the correct answer is option C: zero. The angle between the direction of applied force and displacement should be zero for maximum work.
Work And Energy - Class 9 Science (Compulsory Test) - Question 27

When energy changes from one form to another, the energy that disappears from one form, reappears in exactly equivalents amount in the other form.

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 27
Explanation:
When energy changes from one form to another, the principle of conservation of energy states that the total amount of energy remains constant. This means that the energy that disappears from one form will reappear in exactly equivalent amounts in the other form. This concept is known as energy conservation.
Here is a detailed explanation:
1. Law of Conservation of Energy:
- The law of conservation of energy states that energy cannot be created or destroyed. It can only be transferred or transformed from one form to another.
- This principle is based on the first law of thermodynamics, which is a fundamental principle in physics.
2. Energy Transformation:
- Energy can exist in various forms such as kinetic energy, potential energy, thermal energy, electrical energy, etc.
- When energy changes from one form to another, it undergoes a transformation.
- For example, when a car accelerates, chemical potential energy in the fuel is converted into kinetic energy of the moving car.
3. Conservation of Energy:
- The total amount of energy in a closed system remains constant.
- When energy is transformed from one form to another, the total energy before and after the transformation remains the same.
- This means that the energy that disappears from one form reappears in exactly equivalent amounts in the other form.
4. Energy Conversion Examples:
- When a light bulb is turned on, electrical energy is converted into light and heat energy.
- When a person eats food, chemical potential energy in the food is converted into mechanical energy for the body's movement and thermal energy to maintain body temperature.
- When a battery is used, chemical potential energy in the battery is converted into electrical energy.
5. Practical Applications:
- The principle of energy conservation is used in various fields such as engineering, physics, and environmental science.
- It helps in designing energy-efficient systems and understanding the behavior of energy in different processes.
In conclusion, when energy changes from one form to another, the energy that disappears from one form reappears in exactly equivalent amounts in the other form. This principle is based on the law of conservation of energy, which states that the total amount of energy in a closed system remains constant.
Work And Energy - Class 9 Science (Compulsory Test) - Question 28

In order to get minimum work, the angle between force and displacement should be 90°.

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 28
Statement: In order to get minimum work, the angle between force and displacement should be 90°.
Explanation:
To understand the statement, we need to consider the concept of work done.
1. Work Done:
- Work done is the product of the force applied on an object and the displacement of the object in the direction of the force.
- Mathematically, work done (W) = force (F) × displacement (d) × cos(θ), where θ is the angle between the force and displacement vectors.
2. Angle between Force and Displacement:
- The angle between the force and displacement vectors affects the work done.
- When the angle between the force and displacement vectors is 0°, the work done is maximum.
- When the angle between the force and displacement vectors is 180°, the work done is minimum.
- When the angle between the force and displacement vectors is 90°, the work done is zero.
Explanation of the Statement:
- The statement is incorrect. The angle between force and displacement should be 0° (or cos(0°) = 1) in order to get maximum work, not 90°.
- When the angle is 0°, the force and displacement vectors are in the same direction, resulting in maximum work done.
- When the angle is 90°, the force and displacement vectors are perpendicular to each other, resulting in zero work done.
- In fact, when the angle is 90°, the force applied does not contribute to the displacement of the object.
Conclusion:
- The correct statement should be: "In order to get maximum work, the angle between force and displacement should be 0°."
- Therefore, the given statement is false (option B).
Work And Energy - Class 9 Science (Compulsory Test) - Question 29

When a body falls on the ground and stops, the principle of conservation of energy is violated.

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 29
Explanation:
When a body falls on the ground and stops, the principle of conservation of energy is not violated. The principle of conservation of energy states that energy cannot be created or destroyed but can only be transformed from one form to another. In the case of a falling body, the potential energy is converted into kinetic energy, and when the body comes to a stop, the kinetic energy is converted back into potential energy.
Here is a detailed explanation of why the principle of conservation of energy is not violated when a body falls on the ground and stops:
1. Initial state:
- The body is at a certain height above the ground, possessing potential energy due to its position.
2. Falling:
- As the body falls, it gains kinetic energy and loses potential energy.
- The potential energy is converted into kinetic energy as the body accelerates due to gravity.
3. Impact:
- When the body hits the ground, it comes to a stop.
- The kinetic energy is transferred to the ground, causing deformation and the production of sound.
4. Final state:
- The body is at rest on the ground, with no kinetic energy.
- However, the potential energy is not completely lost; it is converted into other forms such as sound, heat, and deformation.
5. Conservation of energy:
- Throughout the process, the total energy (potential energy + kinetic energy + other forms) remains constant.
- The initial potential energy is equal to the final potential energy plus the energy transferred to other forms.
Therefore, the principle of conservation of energy is not violated when a body falls on the ground and stops. The energy is simply transformed from potential energy to kinetic energy and other forms of energy.
Work And Energy - Class 9 Science (Compulsory Test) - Question 30

When an arrow is released from a bow, potential energy changes into kinetic energy.

Detailed Solution for Work And Energy - Class 9 Science (Compulsory Test) - Question 30
Explanation:
When an arrow is released from a bow, potential energy changes into kinetic energy. This statement is true because of the following reasons:
Potential Energy:
- Potential energy is the energy that an object possesses due to its position or condition.
- In the case of a bow and arrow, when the arrow is pulled back and held in position, it possesses potential energy.
- The potential energy is stored in the bow as elastic potential energy.
Release of the Arrow:
- When the arrow is released, the potential energy is converted into kinetic energy.
- Kinetic energy is the energy of motion.
- As the arrow leaves the bowstring, the potential energy is transferred into the arrow, causing it to move forward.
- The potential energy is converted into kinetic energy, increasing the arrow's speed and giving it the ability to penetrate a target.
Conservation of Energy:
- The conversion of potential energy into kinetic energy follows the principle of conservation of energy.
- According to this principle, energy cannot be created or destroyed; it can only be transferred or transformed from one form to another.
- In the case of the bow and arrow, the potential energy stored in the bow is transformed into kinetic energy in the arrow.
Therefore, when an arrow is released from a bow, the potential energy changes into kinetic energy, making statement A true.
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