Question

A ball of mass $1\, kg $strikes a heavy platform, elastically, moving upwards with a velocity of $5\, m/s.$ The speed of the ball just before the collision is $10\, m/s$ 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 $e=\left[\frac{\left(v_{2}-v_{1}\right)}{\left(u_{1}-u_{2}\right)}\right]_{n}$

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