Question

A ball of mass $0.2\, kg$ is thrown vertically upwards by applying a force by hand. If the hand moves $0.2 \,m$ while applying the force and the ball goes upto $2 \,m$ height further, find the magnitude of the force. (Consider $g=10\, m / s ^{2}$ ).

• A. 4 N
• B. 16 N
• C. 20 N
• D. 22 N

Answer 1

Answer: (D)

Let the velocity of the ball just when it leaves the hand is $u$
then applying, $v^{2}-u^{2}=2 a$ for upward journey
$\Rightarrow -u^{2}=2(-10) \times 2$
$\Rightarrow u^{2}=40$
Again applying $v^{2}-u^{2}=2 a$
for the upward journey of the ball,
when the ball is in the hands of the thrower,
$v^{2}-u^{2}=2 \,a s $
$\Rightarrow 40-0=2(a) 0.2 $
$\Rightarrow a=100 \,m / s ^{2}$
$\therefore F =m a=0.2 \times 100=20 \,N$
$\Rightarrow N -m g=20$
$\Rightarrow N =20+2=22\, N$