## शनिवार, 29 जुलाई 2017

### work and energy class- ix sc.

(Page No.148)
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?

Ans. Since Work (W)

(Page No.149)

1. When do we say that work is done?

Ans. Work is done when a force acts on an object and displacement occurs in the direction of force.

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

Ans.

3. Define 1 J of work.

Ans. 1J is that work which is done when a force of 1N is applied on an object and it moves a distance of 1m in the direction of force.

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?

Ans. Work done

(Page No.152)

1. What is the kinetic energy of an object?

Ans. Kinetic energy of an object is a kind of mechanical energy and it is present in that object due to its state of motion(movement).

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

Ans. Kinetic energy =

where, m = mass of the object, v = velocity of the object(motion)

3. The kinetic energy of an object of mass, m moving with a velocity of 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?

Ans. K.E. of the object =

= 2 kg

If velocity is doubled

= 200/2 = 100 J i.e. K.E. will become four times

If velocity is increased three times

= 225 J i.e. K.E. will become nine times

(Page No.156)

1. What is power?

Ans. 1. Power is the rate of work done. It is denoted by P.

power = work/time

P = W / t

2. Define 1 watt of power.

Ans. If an agent works at the rate of 1 J/s then the power of that agent is 1 watt (where watt is the unit of power).

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

Ans. P = W/t = 1000J/10s = 100 W

4. Define average power.

Ans. An agent may not be able to perform same amount of work in a given period of time always. In other words, power of that agent may vary with time. Hence we can take average power in such situations. Average power is defined as average amount of work done by a body per unit time (i.e. total energy consumed divided by total time).