MadSci Network: Physics
Query:

Re: the physics of a body check in hockey

Date: Mon Feb 25 22:52:50 2002
Posted By: Spencer Bagley, Undergraduate, Physics, Salt Lake Community College
Area of science: Physics
ID: 1013549421.Ph
Message:

Hey AJ, this is going to be a fun question to answer! Here goes!
When something collides with something else, the first thing imparts some of its momentum to the other thing. The amout varies due to the material of the something, just how fast it is going, etc. This may or may not change the motion of the second thing. If it does, the sum of their kinetic energies before and after the collision is constant (no energy is lost to heat). This is called an elastic collision. If the motion of the second thing is unchanged (think about a tomato or Silly Putty hitting a brick wall), the missing energy turns into heat (or possibly light, if they're going really really fast). Also, the two things may stick together and move as one body, like a glob of Silly Putty hitting another glob in midair. This is called an inelastic collision.

What we're concerned about is an elastic collision (well, for now anyway.) Say Player 1 is going to lay the smack down on Player 2. Player 1 weighs, oh, say 50 kilos and he's going at 5 m/s. His momentum is (50)(5)=250 kg*m/s. His kinetic energy is ((50)(5^2))/2=625 J. For the sake of simplicity, Player 2 is at a dead stop, he weighs 50 kilos, and this is a perfectly elastic collision (all of P1's energy is transferred to P2). So after the collision, P1 is at a dead stop and P2 is going 5 m/s, has 250 kg*m/s momentum, and has 625 J of kinetic energy. The force he feels is his mass times his acceleration. His mass is 50 kg and his acceleration is 5 m/s^2. He feels a 250 N force. Ouch.

Let's make things a little more [choose one: fun, complicated]. P2 is going to smack into the boards now. He's still carrying the 5 m/s, 250 kg*m/s, and 625 J from the previous encounter. When he hits the boards, they aren't going to move (we hope). So he's going to feel all his same force directed back at him (that's Newton's 3d law). Again, he weighs 50 kg but since he's going from 5 m/s to 0, his acceleration is -5 m/s^2. This time, he feels a -250 N force. (That negative just indicates that the force is going in the opposite direction than it was last time. Don't leave it out unless your teacher says so because he/she will probably thump on you if you do.)

In short, the board-checked player is going to feel a force equal to his mass times how fast the other player is going (except make sure to stick newtons on the end, or you will be thumped on). Sorry if I got a little windy, but I just love physics. Don't you?

Here's a really nice site with a Java applet showing inelastic and elastic collisions and their effects on velocity, momentum, and kinetic energy.

I hope this helps, and that you get an A on that project of yours. Merry Christmas. :) Spencer Bagley


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