Physics of blocking a Puck

By Rachel Walsh

            For my physics new assignment I chose to evaluate the forces and momentum involved in stopping a puck with your forehead. This was inspired by this block by Ian Lapperriere http://www.youtube.com/watch?v=47ankjpg__Q . First I considered the change in momentum from before and after the hit and then considered the sum of the forces.

Momentum of puck before the block:

Average speed of a slap shot assumed to be 80 mph = 128.8 km/h = 35.8 m/s

  m = 1 pound or 0.454 kg

            p = mv = (35.8m/s)(0.454 kg) = 16.6 kg m/s

Momentum after the block:

v = - 25 m/s (estimated)

            p = mv = (-25 m/s)(0.454 kg) = - 11.35 kg m/s

Sum of forces:

 t of impact = 0.01 seconds

SF = Dp/Dt = (-11.35 kg m/s – 16.6 kg m/s)/(0.01s) = -2795 N

2795 N are applied by the player’s forehead to change the momentum of the puck. Due to Newton’s law stating that every force has an equal and opposite force the puck also applies 2795 N of force on Lapperiere’s forehead. In homework #3 it is stated that 6000 N is needed to fracture the human forehead; therefore, a puck would not be able to fracture the forehead in this example.