Question

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 $65$ $kg$.

Answer 1

Given:

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

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, $m^{\prime }=65$kg

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

Using the first law of motion, the acceleration (a) of the three-wheeler can be calculated as:

$v=u+at$

$a=\frac{(v-u)}{t}=\frac{(0-10)}{4}=-2.5m/s^2$

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

Using Newton's second law of motion, the net force acting on the three-wheeler can be calculated as:

$F=Ma=465\times (-2.5)=-1162.5N$

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