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. What is the retarding force on the vehicle ? The mass of the three-wheeler is 400 kg and the mass of the driver is 65 kg.

• A. $-1162.5N$
• B. $1162.5N$
• C. $1013N$
• D. $-1013N$

Answer 1

The correct option is A $-1162.5N$

Using kinematics equation $v=u+at$
where $v$ and $u$ are final and initial velocities and $a$ is acceleration and $t$ is time in s;
$v=0;u=36km/hr=36\frac{1000m}{60\times 60s}=10m/s(1km=1000m;1hr=60\times 60s);t=4s\Rightarrow a=\frac{-10}{4}=-2.5m/s^2$
From Newton's IInd law of motion $F_{net}=ma$
from given data in question $m=465kg;a=-2.5m/s^2$
$\Rightarrow F=465\times -2.5=-1162.5N$