Energy & Momentum: Slapshot Bobby Hull, one of hockey’s greatest players, could
ID: 1868861 • Letter: E
Question
Energy & Momentum: Slapshot
Bobby Hull, one of hockey’s greatest players, could shoot the puck at the incredible speed of v=120 mph. A standard hockey puck is a vulcanized rubber disc, with the regulation weight of 6 oz (m=170 g). Imagine one of Hull's pucks hits a 200-lb (M=90 kg) goaltender and gets absorbed in his chestpad (meaning the collision is fully inelastic).
a. If the goaltender is initially at rest and we can ignore friction, what are the speeds of the goaltender and the puck after the collision?
b. How much kinetic energy is lost in this collision? This energy presumably has gone into deforming the goalie's gear (and warming it up in the process).
c. Suppose the goalie's pad is 10 cm thick, and it's fully compressed by the puck. What average force does the goalie experience?
Explanation / Answer
a] v = 120 mph = 53.645 m/s
use conservation of linear momentum
mv + 0 = (m+M)V
=> 0.17(53.645) = (0.17 + 90)V
=> V = 0.1011 m/s
this is the speed of the puck-goaltender system after collision
b] Kinetic Energy lost is:
K.Elost = (1/2)mv2 - (1/2)(m+M)V2 = 244.15 J.
C]
V2 = v2 + 2aS
=> (0.1011)2 = 53.6452 + 2a(0.1)
=> a = - 14388.88 m/s2
therefore, the average force experienced by the goalie will be:
F = 90 x 14388.88 = 1294999 N.
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