Question

A ball of mass m moves towards a moving wall of infinite mass with a speed ${}^{\prime }{v}^{\prime }$ along the normal to the wall. The speed of the wall is ${}^{\prime }{u}^{\prime }$ towards the ball. The speed of the ball after 'elastic' collision with wall is:

• A.
  $u+v$ away from the wall
• B.
  $|v-2u|$ away from the wall
• C.
  $|u-v|$ away from the wall
• D.
  $2u+v$ away from the wall

Answer 1

The correct option is D

$2u+v$ away from the wall

Given, speed of the wall towards the ball $v_{WG}=u$
And speed of the ball $=v$
From the frame of wall,
Speed of the ball before collision $=v+u$
As the mass of the wall is infinite,
So, speed of the ball after collision, $v_{BW}=v+u$
Speed of the ball in ground frame,
$v_{BG}=v_{BW}+v_{WG}$
$v_{BG}=(v+u)+u=v+2u$
Final answer: (b)