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All questions of Energy for Grade 6 Exam

The winners of long jump each weighing 56 Kg, 40 Kg and 45 Kg stand on the podium. What is the work done by them on the podium while they receive the medals?
  • a)
    250 J
  • b)
    0 J
  • c)
    282 J
  • d)
    141 J
Correct answer is option 'B'. Can you explain this answer?

Aayush Kumar answered
The winners of long jump each weighing 56kg,40kg and 45 kg stand on podium.Work done by them is 0 Ioule because there is no displacement and if the displacement is 0 then work done will be also 0.

What is the commercial unit of energy?
  • a)
    electron volt
  • b)
    W/s
  • c)
    kW h
  • d)
    joule
Correct answer is option 'C'. Can you explain this answer?

Rajesh Seervi answered
C is correct answer because energy consumed in commercial use in per unit time and which is defined as power and power has unit watt . We take kWh because energy consumed in commercial use have high value so kWh is taken.

A stone weighing 1 kg is dropped from rest from a height of 4 metres above the ground. When it has free-fallen 1 metre its total energy with respect to the ground is
  • a)
    15 J
  • b)
    40 J
  • c)
    20 J
  • d)
    30 J
Correct answer is option 'B'. Can you explain this answer?

**Explanation:**

When an object is dropped from a height, it undergoes free fall. During free fall, the only force acting on the object is the force of gravity. As the object falls, its potential energy is converted into kinetic energy.

**Step 1: Calculate the potential energy at a height of 4 metres.**

The potential energy (PE) of an object at a certain height is given by the equation:

PE = mgh

where m is the mass of the object, g is the acceleration due to gravity, and h is the height.

Given:
m = 1 kg (mass of the stone)
g = 9.8 m/s² (acceleration due to gravity)
h = 4 m (height)

PE = (1 kg)(9.8 m/s²)(4 m) = 39.2 J

So, the potential energy of the stone at a height of 4 metres is 39.2 J.

**Step 2: Calculate the potential energy at a height of 1 metre.**

Using the same formula, we can calculate the potential energy at a height of 1 metre.

Given:
m = 1 kg (mass of the stone)
g = 9.8 m/s² (acceleration due to gravity)
h = 1 m (height)

PE = (1 kg)(9.8 m/s²)(1 m) = 9.8 J

So, the potential energy of the stone at a height of 1 metre is 9.8 J.

**Step 3: Calculate the kinetic energy at a height of 1 metre.**

The kinetic energy (KE) of an object is given by the equation:

KE = 0.5mv²

where m is the mass of the object and v is its velocity.

Since the stone is dropped from rest, its initial velocity is 0 m/s.

Given:
m = 1 kg (mass of the stone)
v = 0 m/s (initial velocity)

KE = 0.5(1 kg)(0 m/s)² = 0 J

So, the kinetic energy of the stone at a height of 1 metre is 0 J.

**Step 4: Calculate the total energy at a height of 1 metre.**

The total energy (TE) of an object is the sum of its potential energy and kinetic energy.

TE = PE + KE

TE = 9.8 J + 0 J = 9.8 J

So, the total energy of the stone at a height of 1 metre is 9.8 J.

Therefore, the correct answer is option B) 40 J.

Work done by a body is
  • a)
    negative, positive or zero
  • b)
    always positive
  • c)
    always zero
  • d)
    always negative
Correct answer is option 'A'. Can you explain this answer?

Anita Menon answered
Positive Work
- If a force displaces the object in its direction, then the work done is positive
So, W=Fd
The example of this kind of work done is motion of ball falling towards ground where displacement of ball is in the direction of force of gravity.

Negative work
- If the force and the displacement are in opposite directions, then the work is said to be negative. For example if a ball is thrown in upwards direction, its displacement would be in upwards direction but the force due to earth’s gravity is in the downward direction.
So here in this case gravity is doing negative work when you throw the ball upwards. Hence the work done by gravitational force is negative. Mathematically when displacement is opposite to the force work done is given by
- Negative work just means that the force and the displacement act in opposite directions.

Case of zero work done
- If the directions of force and the displacement are perpendicular to each other, the work done by the force on the object is zero.
For example, when we push hard against a wall, the force we are exerting on the wall does no work, because in this case the displacement of the wall is d = 0. However, in this process, our muscles are using our internal energy and as a result we get tired.

A freely falling body during its fall will have
  • a)
    Kinetic energy
  • b)
    Potential energy
  • c)
    Sound energy
  • d)
    Both kinetic energy and potential energy
Correct answer is option 'D'. Can you explain this answer?

Jyoti Kapoor answered
Object Falling from Rest. As an object falls from rest, its gravitational potential energy is converted to kinetic energy. Conservation of energy as a tool permits the calculation of the velocity just before it hits the surface. K.E. = J, which is of course equal to its initial potential energy.

A flying aeroplane possesses
  • a)
    only potential energy
  • b)
    only kinetic energy
  • c)
    both potential and kinetic energy
  • d)
    neither potential nor kinetic energy
Correct answer is option 'C'. Can you explain this answer?

Rajat Singh answered
Correct answer is option c because it is flying above ground level it will posses potential energy and since it is moving with the motion associated through it, it possesses kinetic energy.

If the velocity of a moving car is halved, its kinetic energy would
  • a)
    Double
  • b)
    Become Half
  • c)
    Become one fourth
  • d)
    Remain same
Correct answer is option 'C'. Can you explain this answer?

Namrata Desai answered
The kinetic energy of an object is directly proportional to its velocity squared. This means that if the velocity of a moving car is halved, its kinetic energy would decrease significantly.

Explanation:
1. Kinetic Energy Formula: The formula for kinetic energy is KE = 1/2 * m * v^2, where KE is the kinetic energy, m is the mass of the object, and v is the velocity of the object.
2. Halving the Velocity: If the velocity of the car is halved, it means that the new velocity (v') is equal to half of the original velocity (v/2).
3. Calculation: Substituting the new velocity into the kinetic energy formula, we get KE' = 1/2 * m * (v/2)^2 = 1/2 * m * v^2/4 = KE/4.
4. Conclusion: We can see that the new kinetic energy (KE') is equal to one-fourth of the original kinetic energy (KE). Therefore, when the velocity of a moving car is halved, its kinetic energy becomes one-fourth of what it was originally.

Example:
Let's consider an example to illustrate this concept. Suppose a car has a mass of 1000 kg and is initially moving at a velocity of 20 m/s.
- The initial kinetic energy of the car can be calculated using the formula KE = 1/2 * m * v^2 = 1/2 * 1000 kg * (20 m/s)^2 = 200,000 J.
- If the velocity of the car is halved, the new velocity becomes 10 m/s.
- The new kinetic energy can be calculated using the formula KE' = 1/2 * m * (v/2)^2 = 1/2 * 1000 kg * (10 m/s)^2 = 50,000 J.
- Comparing the initial kinetic energy with the new kinetic energy, we can see that the new kinetic energy is one-fourth (1/4) of the original kinetic energy.

Therefore, the correct answer is option C - the kinetic energy would become one-fourth if the velocity of a moving car is halved.

If 1 newton of force displaces a body by 1 m, the work done is
  • a)
    10 joule
  • b)
    5 joule
  • c)
    1 joule
  • d)
    Depends on time
Correct answer is option 'C'. Can you explain this answer?

Prasad Ghoshal answered
Explanation:

Work done is defined as the product of force and displacement in the direction of force. Thus, if 1 newton of force displaces a body by 1 m, the work done will be:

Work = Force x Displacement
= 1 N x 1 m
= 1 joule

Therefore, the correct answer is option 'C'.

If two bodies of different masses have the same K.E. then the relation between momentum and mass will be:
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'D'. Can you explain this answer?

KE = K = 1 /2( mv2
We know, p = mv
⇒ K = p2 / 2m
 p2  =  2Km
As, KE is Same, ie . Constant, momentum would be directly proportional to the square root of mass.

For what value of .ϕ. is work done maximum?
  • a)
    900
  • b)
    00
  • c)
    600
  • d)
    450
Correct answer is option 'B'. Can you explain this answer?

Jatin Basu answered
Work done when a body moves at an angle to the direction of force is W=Fcosϕ×s. The value of Cos is maximum for 00. So, work done will be maximum for this angel.

Find the correct statement
  • a)
    1 Gigawatt is equal to 109W
  • b)
    power is the rate of doing work
  • c)
    both a and b
  • d)
    none of these
Correct answer is option 'C'. Can you explain this answer?

Ashish Saha answered
One Giga watt is equal to 109 watt or 106 kilowatt. The rate of doing work is called as power. Hence both statements are correct.

When we throw a rock from the top of a building which equation describes the energy of the body at each point during the fall?
  • a)
    A
  • b)
    B
  • c)
    C
  • d)
    D
Correct answer is option 'A'. Can you explain this answer?

Mehak answered
It's 'A ' because according to the law of conservation of energy , energy can only be transformed from to form to another , it can neither be created nor destroyed . The total energy before and after the transformation always remains constant.

A fish with weight 35 kg dives and hits the ground (zero height) with kinetic energy equal to 3500J. Find the height through which fish dived. Take g = 10 m/s2
  • a)
    1 km
  • b)
    100 m
  • c)
    10 m
  • d)
    20 m
Correct answer is option 'C'. Can you explain this answer?

Ananya Das answered
We can use the principle of conservation of mechanical energy to solve this problem. According to this principle, the potential energy (PE) at the height is converted into kinetic energy (KE) as the fish dives to the ground.
The formula for potential energy is:
PE=mgh
Where:
  • m=35 kg
  • g=10 m/s^2
  • hhh is the height through which the fish dived (which we need to find)
Given that the kinetic energy when the fish hits the ground is equal to the potential energy it had at the starting height:
KE=PE
3500 J=35×10×h
Now solve for h:
3500=350h
h=3500/350=10 m
So, the height through which the fish dived is 10 meters.
The correct answer is:
3. 10 m.

A force of 10 N displaces a body by 6 m in 3 seconds. The power of the agency applying the force is
  • a)
    1.8 W
  • b)
    5 W
  • c)
    180 W
  • d)
    20 W
Correct answer is option 'D'. Can you explain this answer?

Ananya Das answered
We know, Power :- 
=  force * displacement/Time 
Force is 10 N, displacement = 6 m 
Time = 3 s 
Then, power = 10*6/3 => 10 * 2 ⇒ 20W

Work done by centripetal force is
  • a)
    >0
  • b)
    ≥0
  • c)
    <0
  • d)
    0
Correct answer is option 'D'. Can you explain this answer?

Arvind Singh answered
W = F* d cos (Theta) 

W = Tension * displacement * cos (90) 

The force is perpendicular to the objects motion (or displacement of the object) 

W = T * d * 0 

W= 0

Which of the following is not an example of potential energy?
  • a)
    A compressed spring
  • b)
    Water stored in the reservoir of dam
  • c)
    A stretched rubber band
  • d)
    A moving car
Correct answer is option 'D'. Can you explain this answer?

Aman Majumdar answered
A moving car is an example of Kinetic energy whereas water stored in a dam, compressed springs and stretched rubber band are examples of potential energy.

Assertion (A): Work is not done when a force is applied to an object, but there is no displacement of the object.
Reason (R): According to the scientific definition of work, work is defined as force multiplied by displacement. If there is no displacement, no work is done.
  • a)
    If both Assertion and Reason are true and Reason is the correct explanation of Assertion
  • b)
    If both Assertion and Reason are true but Reason is not the correct explanation of Assertion
  • c)
    If Assertion is true but Reason is false
  • d)
    If both Assertion and Reason are false
Correct answer is option 'A'. Can you explain this answer?

Hadi Al-Ahmed answered
Understanding Work in Physics
In physics, the concept of work is crucial for understanding how forces interact with objects. Let's break down the assertion and reason provided.
Assertion (A):
- Work is not done when a force is applied to an object, but there is no displacement of the object.
Reason (R):
- According to the scientific definition of work, work is defined as force multiplied by displacement. If there is no displacement, no work is done.
Explanation of Assertion and Reason:
- The assertion is true because work requires not just a force but also movement. If an object does not move, regardless of the applied force, no work is accomplished.
- The reason is also true as it accurately reflects the definition of work in physics:
- Work = Force x Displacement
- If displacement is zero, then work is zero, regardless of the force applied.
Conclusion:
- Both the assertion and reason are true. Moreover, the reason directly explains why the assertion is correct. Hence, the correct answer is option 'A' because the reason clarifies the assertion effectively.
Key Takeaway:
- Work involves both force and displacement.
- No displacement means no work is done, even if a force is applied.
Understanding these fundamentals helps in grasping more complex concepts in physics as you progress in your studies.

When two identical bodies are in motion, the body with a higher velocity has __________.
  • a)
    Lower Kinetic Energy
  • b)
    Higher Kinetic Energy
  • c)
    No Kinetic Energy
  • d)
    None of the options
Correct answer is option 'B'. Can you explain this answer?

Imk Pathshala answered
- Kinetic energy is given by the formula  KE = 1/2 mv2
- When two bodies are identical, they have the same mass.
- The kinetic energy depends on the square of the velocity.
- A higher velocity results in a higher K.E. 
- Therefore, the body with a higher velocity has higher kinetic energy.
- Correct answer: B: Higher Kinetic Energy.

Assertion (A): An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line.
Reason (R): The work done by the force of gravity on the object in this scenario is zero.
  • a)
    If both Assertion and Reason are true and Reason is the correct explanation of Assertion
  • b)
    If both Assertion and Reason are true but Reason is not the correct explanation of Assertion
  • c)
    If Assertion is true but Reason is false
  • d)
    If both Assertion and Reason are false
Correct answer is option 'C'. Can you explain this answer?

  • Assertion's Correctness: The assertion that an object follows a curved path and falls back to the ground with the initial and final points on the same horizontal line is correct for a projectile motion scenario.
  • Reason's Correctness: The reason given that the work done by the force of gravity in this situation is zero is incorrect. Gravity does work on the object during its projectile motion, constantly changing its speed and direction.
  • Explanation: In projectile motion, gravity constantly acts on the object, causing it to accelerate towards the ground. This acceleration due to gravity affects the object's path, making it follow a curved trajectory. The work done by gravity is not zero, as it continuously affects the object's motion.
  • Conclusion: While the assertion is true regarding the path of the object, the reason provided is false since gravity does work on the object during its motion. Therefore, Assertion is true, but Reason is false.

In which of the following activities the work is said to be done?
  • a)
    Khushi is pushing a wall of a house but fails to do so
  • b)
    Pinki is walking on a level road with a book on his head.
  • c)
    Shruti is stretching a string.
  • d)
    Harsh is reading a book
Correct answer is option 'C'. Can you explain this answer?

Priya Dasgupta answered
Understanding Work in Physics
In physics, the concept of work is defined based on the force applied and the movement of an object in the direction of that force. Work is only done when a force causes displacement of an object. Let's analyze each option to determine where work is done.
Option A: Khushi Pushing a Wall
- Khushi is applying force on the wall.
- However, since the wall does not move, there is no displacement.
- Thus, no work is done.
Option B: Pinki Walking on a Level Road
- Pinki is walking and carrying a book on her head.
- While she is exerting force to carry the book, the vertical force (upward) is perpendicular to the direction of her movement (horizontal).
- Therefore, no work is done on the book as it does not move in the direction of the force applied.
Option C: Shruti Stretching a String
- When Shruti stretches the string, she applies a force to it.
- As the string extends, there is a displacement in the direction of the force.
- This means that work is done in stretching the string.
- Hence, work is done in this scenario.
Option D: Harsh Reading a Book
- Harsh is not exerting any force that results in the movement of an object in the direction of that force.
- Therefore, no work is done while reading.
Conclusion
Based on the analysis, the correct answer is option 'C' – Shruti is stretching a string, as it involves both force and displacement, thus fulfilling the condition for work to be done.

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