Q2. 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. What is the work
done by the force of gravity on the object?
Since the body returns to a point which is on the same horizontal line through the point of projection, no
displacement has taken place against the force of gravity, therefore, no work is done by the force due to gravity.

Q3. A battery lights a bulb. Describe the energy changes involved in the process.
Within the electric cell of the battery the chemical energy changes into electrical energy. The electric
energy on flowing through the filament of the bulb, first changes into heat energy and then into the light energy.

Q4. Certain force acting on a 20 kg mass changes its velocity from 5 m s-1 to 2 m s-1. Calculate the work
done by the force.

Q5. A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is
horizontal, what is the work done on the object by the gravitational force? Explain your answer.
The work done is zero. This is because the gravitational force and displacement are perpendicular to each
other.

Q6. The potential energy of a freely falling object decreases progressively. Does this violate the law of
conservation of energy? Why?
It does not violate the law Of conservation of energy. Whatever, is the decrease in PE due to loss of height,
same is the increase in the KE due to increase in velocity of the body.

Q7. What are the various energy transformations that occur when you are riding a bicycle?
The chemical energy of the food changes into heat and then to muscular energy. On paddling, the muscular
energy changes into mechanical energy

Q8. Does the transfer of energy take place when you push a huge rock with all your might and fail to move
it? Where is the energy you spend going?
Energy transfer does not take place as no displacement takes place in the direction of applied force. The energy
spent is used to overcome inertia of rest of the rock.

Q9. A certain household has consumed 250 units of energy during a month. How much energy is this in
joules?

Q10. An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy?
If the object is allowed to fall, find its kinetic energy when it is half-way down.
Q11. What is the work done by the force of gravity on a satellite moving round the earth? Justify your
answer.
When a satellite moves round the Earth, then at each point of its path, the direction of force of gravity on the
satellite (along the radius) is perpendicular to the direction of its displacement (along the tangent). Hence, the
work done on the satellite by the force of gravity is zero.

Q12. Can there be displacement of an object in the absence of any force acting on it? Think. Discuss this
question with your friends and teacher.
The answer is both Yes and No. Yes because when an object moves in deep space from one point to another
point in a straight line, the displacement takes place, without the application of force. No, because force
cannot be zero for displacement on the surface of earth. Some force is essential.

Q13. A person holds a bundle of hay over his head for 30 minutes and gets tired. Has he done some work
or not? Justify your answer.
The person does not do work because no displacement takes place in the direction of applied force as the
force acts in the vertically upward direction.

Q14. An electric heater is rated 1500 W. How much energy does it use in 10 hours?
Energy consumed by an electric heater can be obtained with the help of the expression,
P=W/t
where,
Power rating of the heater, P = 1500 W = 1.5 kW
Time for which the heater has operated, t = 10 h
Work done = Energy consumed by the heater
Therefore, energy consumed = Power × Time
= 1.5 × 10 = 15 kWh
Hence, the energy consumed by the heater in 10 h is 15 kWh or 15 units.

Q16. An object of mass, m is moving with a constant velocity, v. How much work should be done on the
object in order to bring the object to rest?
Kinetic energy of an object of mass m moving with a velocity v is given by the expression 1/2mv². To bring the
object to rest, an equal amount of work i.e. 1/2mv² is required to be done on the object.

Q17. Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h.

Q18. In each of the following a force, F is acting on an object of mass, m . The direction of displacement is
from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work
done by the force is negative, positive or zero.
Case I

In this case, the direction of force acting on the block is perpendicular to the direction of displacement.
Therefore, work done by force on the block will be zero.
Case II

In this case, the direction of force acting on the block and the direction of displacement is same.
Therefore, work done by force on the block will be positive.
Case III

In this case, the direction of force acting on the block is opposite to the direction of displacement.
Therefore, work done by force on the block will be negative.

Q19. Soni says that the acceleration in an object could be zero even when several forces are acting on it.
Do you agree with her? Why?
Yes, acceleration in an object could be zero even when several forces are acting on it. This happens when all
the forces cancel out each other i.e., the net force acting on the object is zero.

Q20. Find the energy in kWh consumed in 10 hours by four devices of power 500 W each.
Power rating of each device, P = 500 W = 0.50 kW
Time for which each device runs, t = 10 h
Work done = Energy consumed by each device (E)
We know, power = Energy consumed / Time
Energy consumed by each device= Power × Time
E=Pxt
= 0.50×10 = 5 kWh
Hence, the energy consumed by four devices of power 500 W each in 10 h will be
4 × 5 kWh = 20 kWh = 20 units

Q21. A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?
As the object hits the hard ground, its kinetic energy gets converted into
(i) heat energy (the object and the ground become slightly warm)
(ii) sound energy (sound is heard when the object hits the ground)
(iii) potential energy of configuration of the body and the ground (the object and the ground get deformed a little
bit at the point of collision).