Question

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

Answer 1

Retarding force, $\text F$ = –50 $\text N$
Mass of the body, $\text m$ = 20 $\text {kg}$
Initial velocity of the body, $\text u$ = 15 $\text m/\text s$
Final velocity of the body, $\text v$ = 0
Using Newton’s second law of motion, the acceleration (a) produced in the body can be calculated as:
$\text F$ = $\text {ma}$
–50 = 20 × $\text a$
$\therefore$ $\text a$ = $\frac{-50}{20}$ = -2.5 $\text m/\text s^2$
Using the first equation of motion, the time (t) taken by the body to come to rest can be calculated as:
$\text v$ = $\text u+\text {at}$
∴ $\text t$ = $\frac{-u}{a}$ = $\frac{-15}{-2.5}$ = 6 $\text s$