Page 136 - Understanding NCERT Science 09

• Food grains are getting dried in the sun.
• A sailboat is moving due to wind energy.
2. 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?
3. A battery lights a bulb. Describe the energy changes
involved in the process.
4. Certain force acting on a 20 kg mass changes its velocity
–1
from 5 m s –1 to 2 m s . Calculate the work done by the
force.
5. 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.
6. The potential energy of a freely falling object decreases
progressively. Does this violate the law of conservation of
energy? Why?
7. What are the various energy transformations that occur
when you are riding a bicycle?
8. 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?
9. A certain household has consumed 250 units of energy
during a month. How much energy is this in joules?
10. 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.
11. What is the work done by the force of gravity on a satellite
moving round the earth? Justify your answer.
12. 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.
13. 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.
14. An electric heater is rated 1500 W. How much energy does
it use in 10 hours?
15. Illustrate the law of conservation of energy by discussing
the energy changes which occur when we draw a pendulum
bob to one side and allow it to oscillate. Why does the bob

WORK AND ENERGY 125

Rationalised 2023-24