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 s just in time to save the child What is the average retarding force on the vehicle ? The mass of three wheeler is 400 kg and mass of the driver is 65 kg.

Explanation

Here mass of three-wheeler mi = 400 kg , mass of driver = `m_2` = 65 kg initial speed of auto ,
u = 36 km/h = `36 xx -m/s 10 ms^(-1)` , final speed , v - 0 and t = 4s .
As acceleration , a = v-u / t = 0 - 10 / 4 = - 2.5 `ms^(-2)`
Now F = `(m_1 + m_2)`a = ( 400 + 65 ) `xx` (- 2.5)
= - 1162.5 N = `- 1.2 xx 10^(3) ` N .
The -ve sign shows that the force is retarding force.