## CBSE Physics: IX Work, Energy and Power

12/16/2014 CBSE

Energy

Ability to do work is energy.
If
work is done on the body, energy of the body increases. If workis done by
the body, energy of the body reduces

Ability of a body
to do work due to its motion is called kinetic energy.

There are two ways to
measure kinetic energy of a body moving with velocity v.

(1) A body moving
with velocity v, becomes stationary after some time, if force is
applied opposite to its motion. We can calculate work done during this time
and calculate kinetic energy.

(2) Work required to
be done to make a stationary body, move with velocity v, gives the
value of kinetic energy acquired by it.

Suppose a body of
mass m is lying stationary on a frictionless horizontal surface. When a
force F acts on it, in time t, it undergoes a displacement s, and
acquires velocity v. If work done during this process is W then,

W = F� s W = ma�
s -------------(i)

Also v2 -
u2 = 2as and taking initial velocity u = 0, we get v2
= 2 a s

a s
=1/2v2 -------------------(ii)

From (i) and (ii)

KE = 1/2mv2

when force F acts on
a body moving with initial velocity u if the body acquires velocity v during
displacement s then work done