Work And Energy (Test -1) - Class 9 Science, 21-02-2018 - Class 9 MCQ

When a body moves on a circular path. The force that acts on the revolving body towards the center is called centripetal force. As the force acts as right angle to the direction of motion, so work done is 0.

In physics, a work is said to be done when there is some displacement on the body on which force is applied. Work done is +ve when the body is displaced in direction of applied force.

Potential Energy of a body = mgh, hence the potential energy is maximum when you are standing, given that acceleration due to gravity and mass remain constant.

According to the law of conservation of energy, energy neither be created nor be destroyed, it can be transformed from one form to another. So, all the statements are true.

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.

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.

An aeroplane flying at an altitude possesses both kinetic energy and potential energy. Kinetic energy is due to mass and velocity and potential energy is due to mass and height.

Initial velocity (u)= 0, final velocity (v) =10 m/s, V²−u²= 2gh, 100–0 =2×10×h. h= 100/20 = 5m.

1 kwH =1000 W×60×60 s= 3600000J=  3.6 × 10⁶ J. one watt is equal to one joule per second.

Power of lamp = 100w = 100/1000 = 0.1Kw, time= 5 hrs. Electrical energy = power×time, E =0.1kw×5h= 0.5 kw–h. = 0.5 unit.

The potential energy of a freely falling object decreases progressively due to decrease in height. Kinetic energy is converted into heat and sound energy when a freely falling object on reaching the ground. Work is done only when change in position occurs.

The total energy of the system is always conserved. It is just transformed from one form to other form. Mechanical energy of a system is not conserved. It is converted into other form of energy.

1 watt second (W.S) = 1J/s×s= 1 joule.

No work is done in reading a book, pulling wall of house and walking on a level road with book on his head as there is no change in the position of object on which force is applied. Stretching string causes displacement, so it is a work.

When you compress a coil spring you do work on it. The elastic potential energy increases.

The correct match for each item is as follows:
(1) Battery - (D) Chemical energy converts into electrical energy
(2) Bulb - (A) Electrical energy converts into light energy
(3) Steam engine - (C) Steam energy converts into mechanical energy
(4) Microphone - (B) Sound energy converts into electrical energy
Explanation:
- Battery: A battery is an energy storage device that converts chemical energy into electrical energy. It uses a chemical reaction to produce a flow of electrons.
- Bulb: A bulb or light bulb is an electrical device that produces light when electrical energy is passed through it. The electrical energy is converted into light energy through the process of incandescence.
- Steam engine: A steam engine is a heat engine that uses steam as its working fluid. The steam energy is converted into mechanical energy through the expansion of steam in a piston or turbine.
- Microphone: A microphone is a device that converts sound energy into electrical energy. It does so by using a diaphragm that vibrates in response to sound waves, which then generates an electrical signal.
Therefore, the correct match for each item is (1-D, 2-A, 3-C, 4-B).

1 unit (Kw-H) = 3.6×1063.6×106 joule. Therefore, 250 units = 250×3.6×10⁶joule = 9×10⁸ Joule.

Explanation:
To match the given terms with their correct responses, we need to understand the definitions and units associated with each term.
Definitions:
- Work: Work is the measure of energy transfer that occurs when an object is moved against an external force.
- Power: Power is the rate at which work is done or energy is transferred.
- Force: Force is a push or pull that can cause an object to accelerate or deform.
- Energy: Energy is the ability to do work.
Units:
- Watt (W): Watt is the unit of power and is defined as one joule per second.
- Newton (N): Newton is the unit of force and is defined as the force required to accelerate a mass of one kilogram at a rate of one meter per second squared.
- Kilowatt hour (kWh): Kilowatt hour is the unit of energy and is defined as the amount of energy consumed or produced by a one-kilowatt device over the course of one hour.
- Joule (J): Joule is the unit of work and energy and is defined as the amount of work done when a force of one newton moves an object one meter in the direction of the force.
Matching:
- Work: It is measured in joules (J). Therefore, the correct response for (1) is (D) Joule.
- Power: It is measured in watts (W). Therefore, the correct response for (2) is (A) Watt.
- Power: Kilowatt hour is a unit of energy, not power. Therefore, the correct response for (3) is not provided in the options.
- Force: It is measured in newtons (N). Therefore, the correct response for (4) is (B) Newton.
Matching:
Therefore, the correct response is A: 1-D, 2-A, 3-C, 4-B.

Potential energy of a spring increases when it is in the state of expansion and compression. Elastic potential energy increases with compression and gravitational potential energy increases on expansion.

The gravitational force of the earth on a satellite along a circular path is O as gravitational force of earth acts on right angle which is O for cos.

Electron volt is an unit of energy. One electron volt is equal to 1.6×10⁻¹⁹joule. One mega electron = 1.6×10⁻¹⁹joule×100000=1.6×10⁻¹³joule

To match the given options with the correct responses, we need to understand the units and their corresponding values.
Definitions:
- W: Watt, the SI unit of power
- 1 KW: 1 kilowatt, equal to 1000 watts
- 1 KWh: 1 kilowatt-hour, a unit of energy equal to the energy consumed by a 1 kilowatt device running for 1 hour
- 1 HP: 1 horsepower, a unit of power equal to approximately 746 watts
Now, let's match the options with their correct responses:
Matching:
- (1) W: Watt - Corresponds to option D, 746 W (1 HP is approximately equal to 746 watts)
- (2) 1 KW: 1 kilowatt - Corresponds to option C, 1000W (1 kilowatt is equal to 1000 watts)
- (3) 1 KWh: 1 kilowatt-hour - Corresponds to option B, 3.6×10^6 J (1 kilowatt-hour is equal to 3.6×10^6 joules)
- (4) 1 HP: 1 horsepower - Corresponds to option A, Nm (1 horsepower can be converted to newton-meter)
Therefore, the correct matching is:
- (1) W: 1-D
- (2) 1 KW: 1-C
- (3) 1 KWh: 1-B
- (4) 1 HP: 1-A
Answer: C. 1-D, 2-C, 3-B, 4-A

The kinetic energy of a freely falling object which eventually stops on reaching the ground changes into heat and sound energy.

Momentum = mass x velocity, velocity = momentum/ mass = 16/2 = 8m/s. Kinetic energy = ½ mv2. K.E = ½ x2 x 82 = 64 J.

Potential energy = mgh, here, P.E = 9800 j. mass = 50kg, g=9.8m/s². Therefore, 9800=50×9.8×h, h= 9800/50×9.8= 20m.

To determine the correct unit of energy, we need to understand the definitions of the units provided and their relationship to energy.
A Newton metre:
- A Newton (N) is the unit of force.
- A metre (m) is the unit of distance.
- A Newton metre (Nm) is the unit of torque, which is the product of a force and the distance from the point of rotation.
- Torque is not a unit of energy, so A is incorrect.
A Joule:
- A Joule (J) is the unit of energy in the International System of Units (SI).
- It is defined as the work done when a force of one Newton is applied over a distance of one metre.
- The Joule is the standard unit of energy and is used to quantify various forms of energy such as mechanical, electrical, and thermal energy.
- The Joule is a correct unit of energy, so B is correct.
Kilowatt hours:
- A Kilowatt (kW) is a unit of power, which is the rate at which energy is transferred or converted.
- An hour (h) is a unit of time.
- Kilowatt hours (kWh) is a unit of energy commonly used for electricity consumption.
- It represents the amount of energy consumed or produced when a power of one kilowatt is used for one hour.
- Kilowatt hours is a correct unit of energy, so C is correct.
Kilowatt:
- A Kilowatt (kW) is a unit of power, not energy.
- It is equivalent to one thousand watts and represents the rate of energy transfer or conversion.
- Kilowatt is not a unit of energy, so D is incorrect.
Based on the above explanations, the correct unit of energy is the Joule (B). Therefore, the correct answer is C: (a), (b), and (c) are correct.

Momentum is the product of mass and velocity. P =2×u, u= p/2. Again p =4×v, v = p/4. So, velocity of first 

particle is twice the second. Ratio of kinetic energy =  =(1/2×2×p/2×p/2) /(1/2×4×p/4×p/4)= 2/1 = 2:1.

Displacement is the shortest distance between initial and final position of the body. Displacement may be due to acceleration due to gravity during free fall.

Power = work done / time taken, here, work done = 2000 joule, time = 400 s. power = 2000j/400s = 5 j/s or 5w.

The correct answer is option A: (a) and (b) are correct.

Explanation:

To determine the correct option, let's analyze each statement:

• A: It is necessary to do the work




• This statement implies that work needs to be done.




• B: A force is acted on the object and the object is displaced




• This statement implies that when a force is applied to an object and the object undergoes displacement, work is done.


• Work is defined as the product of force and displacement in the direction of the force.


• Therefore, when a force is applied to an object and it undergoes displacement, work is done on the object.




• C: A force is acted on an object, but it is not displaced




• This statement implies that a force is applied to an object but the object does not undergo any displacement.


• In this case, no work is done on the object because there is no displacement in the direction of the force.

Based on the analysis, it can be concluded that statement A and B are correct, while statement C is incorrect. Therefore, the correct option is A: (a) and (b) are correct.