Chapter-11. Work and Energy Science class 9 in english Medium CBSE Notes

Kinetic Energy: The energy possessed by an object due to its motion is called kinetic energy.

Example of Kinetic Energy: 

(i) A falling coconut

(ii) a speeding car

(iii) a rolling stone

(iv) a flying aircraft

(v) flowing water

(vi) blowing wind

(vii) a running athlete

(viii) A moving cricket ball

Main-points:

• A moving object can do work.
• An object moving faster can do more work than an identical object moving relatively slow.
• The kinetic energy of a body moving with a certain velocity is equal to the work done on it to make it acquire that velocity.

Formula for Kinetic Energy:

Consider an object of mass, 'm' moving with a uniform velocity, 'u'. Let it now be displaced through a distance 's' when a constant force, F acts on it in the direction of its displacement. 

Its velocity changes from 'u' to 'v'. Then accelaration is 'a'.

Work done, W = f × s                  ......... (i) 

and              f = ma                      ......... (ii) 

Now           W = ma × s               .......... (iii) 

According to third equation of motion 

                2as =  v² - u² 

So,              s =  v² - u² / 2a          ......... (iv) 

Now putting the value of s from (iv) in equation (iii) 

if u = 0 (When the body or an object starts moving from rest)