Question

The velocity of a body of mass $20kg$ decreases from $20{ms}^{-1}\text{to}5{ms}^{-1}$ in a distance of $100m$. Force on the body is

• A. $27.5N$
• B. $47.5N$
• C. $37.5N$
• D. $67.5N$

Answer 1

Answer: (C)

$37.5N$

Given,

Initial velocity, $u=20m/s$

Final velocity, $v=5m/s$

Distance travelled, $s=100m$

Using the 3rd equation of motion,

$v^2-u^2=2as$

where $a$ is the acceleration

$5^2-{20}^2=2a\times 100$

$25-400=200a$

$-375=200a$

$a=-\frac{375}{200}=-1.875$

So, the acceleration is $-1.875m/s^2$

Now,

Force = mass $\times$ acceleration

$=20kg\times -1.875m/s^2$

$=-37.5kgm/s^2$

$=-37.5N$

(Negative sign indicates that the force is opposite to the direction of motion.)

Therefore, the force on the body is $-37.5N$