On a level road, a scooterist applies brakes to slow down from a speed of 10m/s to 5 m/s. If the mass of the scooterist and the scooter be 150 kg, calculate the work done by the brakes. (Neglect air resistance and friction).

Asked by rajneesh kumar | 29th Nov, 2013, 01:30: PM

Expert Answer:

Method 1
 

As per work-kinetic energy theorem , change in kinetic energy resulting from application of external forces is equal to work done.

Here m1 = m2 = m = mass of the scooterist + mass of scooter = 150 kg
 
 
W= Ek1 - Ek2
=
= 1/2 x 150 x (10)2 -  1/2 x 150  x (5)2
 
= 5625J
 
 
 
Method 2

u = 10 m/s

v = 5 m/s

total mass = 150 kg

third eq. of motion :- v2 - u2 = 2as

s = (v2 - u2) / 2a ...... (1)

W = Fs .......(2)

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

 

from (1),(2),(3)

W = ma  x (v2 - u2) / 2a

From this method also you will get same answer.

Answered by Komal Parmar | 29th Nov, 2013, 02:38: PM

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