A 55 kg skater at rest on a frictionless rink throws a 3 kg ball, giving the ball a velocity of 8 m/s. What is the velocity of the skater immediately after?

Velocity's direction will be in opposite to direction of ball.

Explanation:

Momentum = 8m/s,

using law of conservation of momentum;

              50v + 20 = 0 (as initial momentum is zero)

or

p = m/v (m is mass and v is velocity)

v = p/m  (p is momentum)

putting values;

v = 20/50

  = 0.4 m/s

it proves that direction of velocity is opposite direction of ball.