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Q.

A truck of mass 1800 kg is moving with a speed 54 km/h. When brakes are applied, it stops with uniform negative acceleration at a distance of 200 m. Calculate the force applied by the brakes of the truck and the work done before stopping.
 


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a

200 kJ

b

202.5 kJ

c

151.75 kJ

d

101.25 kJ 

answer is B.

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Detailed Solution

A truck of mass 1800 kg is moving with a speed 54 km/h. When brakes are applied, it stops with uniform negative acceleration at a distance of 200 m. The force applied by the brakes of the truck and the work done before stopping is 202.5 kJ.
Given:
Mass, m=1800 kg
Initial velocity, u=54 km/h
       =54×518m/s
       =15 m/s
Final velocity, v=0
Distance, s=200 m
from the 3rd equation of motion
v2-u2=2asRetardation, a=v2-u22s
  =0-(15)22×200
         a=-916m/s2
Force, F=ma
    =1800×-916
           F=-1012.5 N
The negative sign indicates force acts in the opposite direction to motion.
Work done =Fs
 =-1012.5 × -200
 =202500 J
   =202.5 kJ
 
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