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**Page 148**

**Q.1 A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force. Let ****us take it that the force acts on the object through the displacement. What is the work done in this case? **

then the work done W on the body by the force is given by:

Work done = Force × Displacement

W = F × S

Where,

F = 7 N

S = 8 m

Therefore, work done, W = 7 × 8

= 56 Nm

= 56 J

**Page 149**

**Q.1 When do we say that work is done?**

* Sol.* Work is done whenever the given conditions are satisfied:

**Q.2 Write an expression for the work done when a force is acting on an object in the direction of its ****displacement.
**

Work done = Force × Displacement

W = F × s8080

**Q.3 ****Define 1 J of work.
**

**Q.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 = Force × Displacement

W = F × d

Where,

Applied force, F = 140 N

Displacement, d = 15 m

W = 140 × 15 = 2100 J

Hence, 2100 J of work is done in ploughing the length of the field.

**Page 152**

**Q.1 What is the kinetic energy of an object?**

* Sol. *Kinetic energy is the energy possessed by a body by the virtue of its motion. Every moving object possesses kinetic energy. A body uses kinetic energy to do work. Kinetic energy of hammer is used in driving a nail into a log of wood, kinetic energy of air is used to run wind mills, etc.

**Q.2 Write an expression for the kinetic energy of an object.**

* Sol.* If a body mass m is moving with a velocity v, then its kinetic energy is given by the expression,

. Its SI unit is Joule (J).

**Q.3 The kinetic energy of an object of mass, m moving with a velocity of 5 m s−1 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?**

* Sol. *Expression for kinetic energy

m = Mass of object

v = Velocity of the object

Given that kinetic energy,

** (i)** If the velocity of an object is doubled, then v = 5 × 2 = 10 .

Therefore, its kinetic energy becomes 4 times its original value, because it is proportional to the square of the velocity. Hence, kinetic energy = 25 × 4 = 100 J.

** (ii)** If velocity is increased three times, then its kinetic energy becomes 9 times its original value, because it is proportional to the square of the velocity. Hence, kinetic energy = 25 × 9 = 225 J.80

**Page 156**

**Q.1 What is power?**

* Sol. *Power is the rate of doing work or the rate of transfer of energy. If W is the amount of work done in time t, then power is given by the expression,

It is expressed in watt (W).

**Q.2 ****Define 1 watt of power:**

* Sol.* A body is said to have power of 1 watt if it does work at the rate of 1 joule in 1 s,i.e.,

**Q.3 A lamp consumes 1000 J of electrical energy in 10 s. What is its power?
**

Work done = Energy consumed by the lamp = 1000 J

= 100 W

**Q.4 Define average power.
**

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