The driver of a three-wheeler moving with a speed of 36 km/h sees a child standing in the middle of the road and brings his vehicle to rest in 4.0 s just in time to save the child. What is the average retarding force on the vehicle? The mass of the three-wheeler is 400 kg and the mass of the driver is 65 kg.  from Physics Laws of Motion Class 11 Manipur Board
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The driver of a three-wheeler moving with a speed of 36 km/h sees a child standing in the middle of the road and brings his vehicle to rest in 4.0 s just in time to save the child. What is the average retarding force on the vehicle? The mass of the three-wheeler is 400 kg and the mass of the driver is 65 kg. 

Initial speed of the three-wheeler, u = 36 km/h = 10 m/s

Final speed of the three-wheeler, v = 0 m/s

Time, t = 4 s

Mass of the three-wheeler, m = 400 kg

Mass of the driver, m' = 65 kg

Total mass of the system, M = 400 + 65 = 465 kg

According to the first law of motion,

Acceleration (
a) of the three-wheeler is, 

                       v = u + at 

Therefore,

 
a = fraction numerator left parenthesis straight v space minus space straight u right parenthesis over denominator straight t end fraction space equals space fraction numerator left parenthesis 0 minus 10 right parenthesis over denominator 4 end fraction space equals space minus space 2.5 space straight m divided by straight s squared

The negative sign indicates that the velocity of the three-wheeler is decreasing with time.

Now, using Newton’s second law of motion, the net force acting on the three-wheeler is, 

       F = Ma

         = 465 × (–2.5)
 
         = –1162.5 N 

The negative sign indicates that the force is acting against the direction of motion of the three wheeler. 
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