Question

A ball of mass $1kg$strikes a heavy platform, elastically, moving upwards with a velocity of $5m/s.$ The speed of the ball just before the collision is $10m/s$ downwards. Then the impulse imparted by the platform on the ball is

• A. 15 N-s
• B. 10 N-s
• C. 20 N-s
• D. 30 N-s

Answer 1

Answer: (D)

As we know $e={\left[\frac{\left(v_2-v_1\right)}{\left(u_1-u_2\right)}\right]}_n$


$1=\frac{(-5)-\left(v_1\right)}{(10)-(-5)}$
$\Rightarrow v_1=-20m/s$
Hence ball will start moving towards upward direction with velocity $20m/s$.
Let impulse imparted by plate force on the ball is $J$ in upward direction.
$m{u}_1-J=-m{v}_1$
$\Rightarrow J=m\left(u_1+v_1\right)$
$=1(10+20)=30kgm/s$