Physics Exercise – Work, Energy And PowerPhysics Solutions: Work, Energy And Power
1. When do we say that work is done?
Work is said to be done when a force is applied on an object and it displaces in the direction of the applied force. If there is no displacement, no work is done, even if force is applied.
2. Write an expression for the work done when a force is acting on an object in the direction of its displacement.
The expression for work done (W) when a force (F) is applied and the object displaces a distance (d) in the direction of the force is:
W = F × d × cos(θ)
where θ is the angle between the force and the direction of displacement.
3. Define 1 J of work.
1 Joule (J) of work is done when a force of 1 Newton (N) displaces an object by 1 meter in the direction of the force. Mathematically, 1 J = 1 N × 1 m.
4. A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field?
Work done (W) is calculated using the formula:
W = F × d
Here, F = 140 N and d = 15 m.
W = 140 × 15 = 2100 J.
So, 2100 Joules of work is done in ploughing the field.
5. What is the kinetic energy of an object?
Kinetic energy is the energy possessed by an object due to its motion. It is given by the formula:
K.E. = (1/2) m v²
where m is the mass of the object, and v is its velocity.
6. Write an expression for the kinetic energy of an object.
The expression for kinetic energy (K.E.) of an object is:
K.E. = (1/2) m v²
where m is the mass of the object and v is its velocity.
7. The kinetic energy of an object of mass, m moving with a velocity of 5 m/s is 25 J. What will be its kinetic energy when its velocity is doubled? What will be its kinetic energy when its velocity is increased three times?
If the velocity is doubled (v = 10 m/s), then kinetic energy becomes:
K.E. = (1/2) m (10)² = 100 J
If the velocity is tripled (v = 15 m/s), then kinetic energy becomes:
K.E. = (1/2) m (15)² = 225 J
8. What is power?
Power is the rate at which work is done or energy is transferred. The formula for power (P) is:
P = W / t
where W is work done and t is the time taken.
9. Define 1 watt of power.
1 watt is defined as the power consumed when 1 joule of work is done in 1 second. Mathematically, 1 watt = 1 joule/second.
10. A lamp consumes 1000 J of electrical energy in 10 seconds. What is its power?
The power (P) consumed by the lamp is given by:
P = W / t
Here, W = 1000 J, and t = 10 s.
P = 1000 / 10 = 100 W.
So, the power consumed is 100 watts.
11. Define average power.
Average power is the total work done divided by the total time taken. It is given by:
Average power = Total work done / Total time.
Physics Exercise – Work, Energy And Power
1. Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term ‘work’:
- Suma is swimming in a pond: Work is done because displacement occurs.
- A donkey is carrying a load on its back: No work in the vertical direction, but work is done in the horizontal direction if it moves.
- A wind-mill is lifting water from a well: Work is done as water is displaced upward.
- A green plant is carrying out photosynthesis: No mechanical work is involved.
- An engine is pulling a train: Work is done as the train moves.
- Food grains are getting dried in the sun: No mechanical work is involved.
- A sailboat is moving due to wind energy: Work is done due to displacement.
2. An object thrown at a certain angle moves in a curved path. What is the work done by gravity?
The work done by gravity is zero because the displacement in the vertical direction is zero.
3. A battery lights a bulb. Describe the energy changes involved.
Chemical energy in the battery is converted into electrical energy, then to light and heat energy in the bulb.
4. A 20 kg mass changes its velocity from 5 m/s to 2 m/s. Calculate the work done by the force.
Initial Kinetic Energy (KE1) = 250 J
Final Kinetic Energy (KE2) = 40 J
Work done = KE1 – KE2 = 210 J
5. A mass of 10 kg is moved horizontally. What is the work done by the gravitational force?
The work done is zero because gravity acts vertically, and there’s no vertical displacement.
6. The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy?
No, because the potential energy is converted into kinetic energy. The total energy remains constant.
7. What are the various energy transformations when riding a bicycle?
Chemical energy from food is converted into mechanical energy for pedaling, and some of it is transformed into heat due to friction.
8. Does the transfer of energy take place when you push a huge rock but fail to move it? Where does the energy go?
Yes, energy is spent, but no work is done on the rock. The energy is converted into heat in your body.
9. A household consumes 250 units of energy. How much is this in joules?
250 units of energy = 250 kWh = 9 x 108 joules.
10. A 40 kg object is raised 5 m above the ground. Find its potential energy and its kinetic energy halfway down.
Potential Energy = 1960 J
Halfway Kinetic Energy = 980 J
11. What is the work done by gravity on a satellite moving around Earth?
The work done by gravity is zero because the force of gravity is perpendicular to the displacement.
12. Can displacement occur without any force acting on the object?
Yes, an object can move at constant velocity without external force (Newton’s First Law of Motion).
13. A person holds a bundle of hay over his head for 30 minutes. Has he done any work?
No work is done because there is no displacement in the direction of the force.
14. How much energy does a 1500 W heater consume in 10 hours?
Energy consumed = 54 x 106 J
15. Explain the energy changes in a pendulum bob. Why does it eventually come to rest?
The potential energy converts to kinetic energy and vice versa. It eventually comes to rest due to friction converting mechanical energy into heat.
16. How much work is needed to bring a moving object to rest?
The work required is equal to the object’s initial kinetic energy: W = ½ mv².
17. Calculate the work to stop a car of 1500 kg moving at 60 km/h.
Work required = 208,645.83 J
18. Is the work done by the force positive, negative, or zero?
This depends on the direction of force relative to displacement: positive if they are in the same direction, negative if opposite, and zero if perpendicular.
19. Can acceleration be zero if multiple forces are acting on an object?
Yes, if the net force is zero, the object’s acceleration will also be zero.
20. Find the energy in kWh consumed in 10 hours by four devices of power 500 W each.
Energy consumed = 20 kWh
21. A freely falling object stops upon reaching the ground. What happens to its kinetic energy?
The kinetic energy is converted into other forms such as heat, sound, and deformation energy.
1. What is the difference between kinetic and potential energy?
Kinetic energy is the energy an object has due to its motion, while potential energy is the energy an object has due to its position or configuration.
2. How is work related to force and displacement?
Work is the product of force and displacement in the direction of the force. Mathematically, W = F × d × cos(θ).
3. What is the work done if a force is applied but there is no displacement?
If there is no displacement, the work done is zero, even if a force is applied.
4. How does friction affect the work done on an object?
Friction opposes motion, so it reduces the net work done on an object. In many cases, friction results in negative work.
5. What is mechanical advantage?
Mechanical advantage is the ratio of the output force to the input force in a machine. It measures how effectively a machine amplifies the applied force.
6. What is meant by the term “energy transformation”?
Energy transformation refers to the process of changing energy from one form to another, such as kinetic energy converting to potential energy or chemical energy converting to electrical energy.
7. What is the significance of the conservation of energy?
The law of conservation of energy states that energy cannot be created or destroyed, only transferred or transformed. This principle is fundamental to understanding energy interactions in physical systems.
8. How is power different from energy?
Power is the rate at which energy is transferred or work is done. Energy is the capacity to do work, while power measures how quickly the work is done.
9. Can an object have mechanical energy even if it is not moving?
Yes, an object can have potential energy (a type of mechanical energy) even if it is not moving. For example, a rock at the top of a hill has potential energy due to its position.
10. Why is no work done when an object moves in a circular path under the influence of a centripetal force?
In circular motion, the centripetal force is always perpendicular to the direction of displacement. Since work depends on the component of force in the direction of displacement, no work is done by the centripetal force.
