Question

A 80.0 kg ice hockey goalie, originally at rest, catches a 0.150 kg hockey puck slapped at him at a velocity of 34.0 m/s. Suppose the goalie and the ice puck have an elastic collision and the puck is reflected back in the direction from which it came. What would their final velocities (in m/s) be in this case? (Assume the original direction of the ice puck toward the goalie is in the positive direction. Indicate the direction with the sign of your answer.)

Answer 1

Use the 1D elastic-collision formulas (goalie mass M = 80.0 kg, puck mass m = 0.150 kg, puck initial v = +34.0 m/s, goalie initial v = 0):

v_puck,f = (m - M)/(m + M) * v_puck,i = (0.150 - 80.0)/(80.15) * 34.0 ≈ -33.9 m/s
v_goalie,f = (2 m)/(m + M) * v_puck,i = (2·0.150)/(80.15) * 34.0 ≈ +0.127 m/s

So the puck is reflected back at about −33.9 m/s and the goalie moves forward at about +0.127 m/s.