# SSC Worksheet for chapter6 Energy : The Driving Force class 9

## Fill in the blanks:

1. By stretching the string of the bow, the archer stores ________ energy in it.
2. 1 J = ________ erg.
3. When a stone is tied to a string and whirled in a circular path, string does ________ work on stone.
4.  ________ is the capacity to do work.
5. Energy possessed by a body by virtue of its motion is ________ energy.
6. Energy possessed by a body because of its shape or position or configuration is known as ________ energy.
7. 1 KWH = ________ Joules.
8.  If velocity of a body doubles, its kinetic energy will ________ .
9. Why are dams are constructed on rivers?
10. Why is it easy to swim in a swimming pool then in a river?
11. Show that the sum of potential energy + kinetic energy of a freely falling body remain constant during its motion.
12. Write a note on Power.
13. Explain with examples law of conservation of energy.
14. Derive an expression for potential energy of a body of mass m which is raised to a height h above surface of earth.
15. A force of 20N acts on a body. If the displacement of body is 0.6 m in the direction opposite to that of the force, calculate work done by force.
16.  A man carrying a box of mass 20 kg climbs to a height of l0 m in 40 s. Calculate power delivered by him to the box. (g = 9.8 m/s2).
17. An electric bulb of 40 W is used for 50 days at 5 hours per day. Find units of energy consumed by bulb. Find also cost of using electricity at rate of Rs. 3 per unit.

### Solutions

1. Potential 2.107 3.Zero 4. Energy 5. Kinetic

6. Potential 7. 3.6 ´ 106J 8.Double 15. –12 J 16. 49 W

17. 10 units, Rs. 30

9. Because when a dam is built on a river, the nearby accumulated water is called lake. The water gains a huge amount of potential energy on being accumulated and due to its height. When released in small amounts, the water's pot. Energy converts to kinetic energy which turns the turbine wings. Which further move the coil within 2 strong magnets, which creates emf in the coil. Which results in the formation of electric current?

10. In a river, flowing water exerts a force on the swimmer. To cross the river or to swim upstream the swimmer has to overcome this resistive force. This requires spending a lot of energy.
As there is no flow of water in a swimming pool, the swimmer does not have to overcome the resistive force as in river. Hence it is easier to swim in swimming pool than in a river.

11. Let a body of mass 'm' placed at a height 'h' above the ground, start falling down from rest.

In this case we have to show that the total energy (potential energy + kinetic energy) of the body at A, B and C remains constant i.e, potential energy is completely transformed into kinetic energy.

##### Body of Mass 'm' Placed at a Height 'h'

At A,

Potential energy = mgh

Kinetic energy

Kinetic energy = 0 [the velocity is zero as the object is initially at rest]

Total energy at A = Potential energy + Kinetic energy

= mgh + 0

Total energy at A = mgh …(1)

At B,

Potential energy = mgh

= mg(h - x) [Height from the ground is (h - x)]

Potential energy = mgh - mgx

Kinetic energy = 1/2 mv2

The body covers the distance x with a velocity v. We make use of the third equation of motion to obtain velocity of the body.

v2 - u2 = 2aS

Here, u = 0, a = g and S = x

v2 - u2 = 2as

Kinetic energy = 1/2 mv2

v2= 2gx

Kinetic energy = mgx

Total energy at B = Potential energy + Kinetic energy

= mgh +0.mgx

Total energy at B = mgh …(2)

At C,

Potential energy = m x g x 0 (h = 0)

Potential energy = 0

Kinetic energy = 1/2 mv2

The distance covered by the body is h

v2 - u2 = 2aS

Here, u = 0, a = g and S = h

v2 - 0 = 2gh

v2 = 2gh

Kinetic energy = 1/2 mv2

= 1/2 m x 2gh

Kinetic energy = mgh

Total energy at C = Potential energy + Kinetic energy

= 0 + mgh

Total energy at C = mgh …(3)

It is clear from equations 1, 2 and 3 that the total energy of the body remains constant at every point. Thus, we conclude that law of conservation of energy holds good in the case of a freely falling body.

12. Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is thewatt, onejoule per second.

13. The law of conservation of energy is that energy cannot be created or destroyed, but it can be transformed from one form to another (including transformation into or from mass, as matter). The total amount of energy in a closed system never changes.

Example: When water is used to produce electricity. In this process, firstly, water is allowed to fall from great heights, so potential energy is converted to kinetic energy(K.E.). Then this energy is used to rotate the turbine of a generator which produces electricity. Thus potential energy of water in a dam is converted into kinetic energy which is then converted to electric energy.

14. PE= Energy stored during work

= Force x displacement=(mg)h=mgh.joules

15. ω = -F x d = (20 x 0.6) = -12 joules

16. p = ω/t = mgh/t = 20 x 9.8 x 10 / 40 = 49watts

17. p = 40ω = 40 x 10-3 kω

T = 50 x 5 = 250hrs

1 unit = 1kωh

Units covere = P x T = 40 x 10-3 (kω) x 250h

= 10kωh = 10units

Cost = 10 x 3 = 30 rupess

For other SSC Worksheet for class 9 Science check out the main page of Physics Wallah