Question

A constant retarding force of $50N$ is applied to a body of mass $20kg$ moving initially with a speed of $15m{s}^{-1}$. How long does the body take to stop?

• A. $6s$
• B. $4s$
• C. $7s$
• D. $2s$

Answer 1

The correct option is A $6s$

From Newton's IInd law of motion $F_{net}=ma$
From given data in question $F=-50N;m=20kg;u=15m/s;v=0m/s$
$\Rightarrow a=\frac{-50}{20}=-2.5m/s^2$
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;
$v=0;u=15m/s;a=-2.5m/s^2\Rightarrow t=\frac{15}{2.5}=6s$