Class 9 Science

Question 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?

**Answer:** Given; F = 7N and displacement s = 8 m

Work Done = Force x Displacement

`= 7 N xx 8 m = 56 Nm` or 56 J

Question 2. When do we say that work is done?

**Answer:** When a force is applied on a body and displacement happens in the body along the direction of the force applied, we say that work is done.

Question 3. Write an expression for the work done when a force is acting on an object in the direction of its displacement.

**Answer:** Work Done = Force x Displacement
Or, `W = F xx s`

Question 4. Define 1 J of work.

**Answer:** When a force of 1 N causes displacement of 1 m in an object in the direction of force applied; work done is 1 J.

Question 5. 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?

**Answer:** Given; F = 140 N and s = 15 m

`W = F xx s`

`= 140 N xx 15 m = 2100 Nm`

Question 6. What is the kinetic energy of an object?

**Answer:** The energy possessed by a moving object because of its velocity is called kinetic energy.

Question 7. Write an expression for the kinetic energy of an object.

**Answer:** If m is the mass and v is the velocity of an object, kinetic energy is given by following equation.

`E_k = ½ mv^2`

Question 8. 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?

**Answer:** We know that `E_k = ½ mv^2`

This means that kinetic energy varies directly as square of velocity and directly as mass.

When velocity becomes double, the kinetic energy will be 2^{2} times, i.e. 4 times.

So, kinetic energy in this case `= 25 J xx 4 = 100 J`

When velocity becomes triple, the kinetic energy will be 3^{2} times, i.e. 9 times.

So, kinetic energy in this case `= 25 J xx 9 = 225 J`

Question 9. What is power?

**Answer:** The rate of doing work or rate of transfer of energy is called power. If W is the work done in time t; then

`P = W/t`

Power is expressed in Watt (W).

Question 10. Define 1 watt of power.

**Answer:** When 1 J of work is done in 1 s then power is 1 W.

Or, `1 W = (1 J)/(1 s)`

Question 11. A lamp consumes 1000 J of electrical energy in 10 s. What is its power?

**Answer:** Given; W = 1000 J and t = 10 s

`P = W/t`

`= (1000 J)/(10 s) = 100 W`

Question 12. Define average power.

**Answer:** Total work done by an object in total time is called average power.

Average power = total work done/total time taken

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