Work and energy - Class 9

 1:- What is work?

Ans:- Work is said to be done when we applied a force upon an object and the object displace from their mean position in any direction.   

It is a scaler quantity. It is denoted by 'W'.

Work done by a force on a body depends on two factors

👉 magnitude of force

👉Displacement through which the body moves 

W ∝ F

W ∝ s

So, W ∝ Fs

And, W = k Fs

The value of k in SI. unit is '1'. 

So, W = Fs

Work = Force × displacement

The SI. unit of force = Newton (N)

The SI. unit of displacement = metre (m)

Therefore SI. unit of work = Nm

It is also denoted by Joule (J)

     1Nm = 1 J

2:- What is 1 Joule work?

Ans:- 1 Joule work is the amount of work done on an object when a force of 1N displace it by 1m along the line of action of the force. 

 1 Joule = 1 N × 1m

Notes:- Mental labour is not considered work as per science.

3:- What is the formula of work done If work and displacement are in same direction?

Ans:- If work and displacement are in same direction then work done is positive.

In this condition, the formula of work done is

W = Fs

3:- What is the formula of work done If work and displacement are in opposite direction?

 Ans:- If work and displacement are in opposite direction then work done is negative.

In this condition, the formula of work done is

W = - Fs

4:- When is the work done become '0'?

Ans:- If displacement is perpendicular to the force than work done is '0'.

 

Energy

1:- What is energy?

Ans:- The capacity of the body to do work is called energy of body.

 SI. unit of energy is Joule and it is denoted by 'J'.

              1KJ =  1000J 

2:- How many forms of energy?

Ans:- There are various forms of energy.

(1) Potential energy

(2) Kinetic energy

(3) Heat energy

(4) Chemical energy

(5) Electrical energy

(6) Light energy

3:- What is potential energy?

Ans:- Energy possessed by a body due to its position or shape is called potential energy.

       Ex:-

4:- What is kinetic energy? Write formula for kinetic energy.

Ans:- Energy possessed by a body due to its motion is called kinetic energy. Kinetic energy of an object increases with increasing its speed.  

Let an object of mass 'm', moves with uniform velocity 'u', the object displace 's', due to constant force 'F'.   Than work done = Force × displacement

                                   = F×s  ........... (1)

Due to force the velocity changes to 'v' and acceleration produced is 'a'.

So, relation between 'v', 'u', 's' and 'a' is. 

V² - u² = 2as

s =  V² - u²/2a ........... (2)

and F = ma ..............(3)