An ice hockey puck slides along the ice at 12 m/s. A hockey stick delivers an impulse of 4.0 kg * m/s causing the puck to move off in the opposite direction with the same speed.

What is the mass of the puck?

Known variables:

initial velocity (v1) : 12 m/s

final velocity (v2) : -12 m/s   (opposite direction)

impulse: 4 kg * m/s
Impulse is the change in momentum

Ft = mv1-mv2
Ft = m(v1-v2)
4 = m [12-(-12)]
-24m = 4

Solve for m:
m = 1/6   or

m = .17 kg