Question

A block of mass 8 kg is moving with a velocity of 36 km/h. If its velocity is reduced to 18 km/h in 0.5 seconds, the retarding force on it is

• A. 32 N
• B. 80 N
• C. 120 N
• D. 64 N

Answer 1

The correct option is B 80 N

Given,

$m=8kg$

$u=36km/h=\frac{36\times 1000}{60\times 60}=10m/s$

$v=18km/h=\frac{18\times 1000}{60\times 60}=5m/s$

$t=0.5sec$

1st equation of motion,

$v=u+at$

$5=10+0.5a$

$a=-\frac{5}{0.5}=-10m/s^2$

Retarding force,

$F=ma=-8\times 10=-80N$

The correct option is B.