# Elastic Collision - Ice Hockey

## Homework Statement

A HOCKEYPLAYER SKATES AT 7.85 M/S AND HAS A MASS OF 120 KG WHEN HE COLLIDES WITH ANOTHER STAIONARY HOCKEY PLAYER. THE SECOND HOCKEY PLAYER IS NOW MOVING AT 5.67 M/S. FIND THE MASS OF THE SECOND HOCKEY PLAYER AND THE SPEED OF THE FIRST. ASSUME THIS COLLISION IS ELASTIC.

## Homework Equations

[Rho]playerone + [Rho]player2 == [Rho]playeroneafter + [Rho]player2after
^^^ COnservation of Momentum
And then Another Equation Which Escapes me.

## The Attempt at a Solution

Find: Pplayer1, Pplayer2,

How do I find the Mass and Speed?

I have been doing Collision Problems for some time now, but must have gotten hit too hard in hockey, because I am lost!

Many Thanks from Frigid CT

Related Introductory Physics Homework Help News on Phys.org
Andrew Mason
Homework Helper
What does it mean when the collision is "elastic"?

AM

cepheid
Staff Emeritus
Gold Member
Welcome to PF papereater33!

You have two unknowns. One is the mass of the second player. The other unknown is the speed of player 1 after the collision.

Using conservation of momentum, you can relate these two unknowns to each other (i.e. solve for one in terms of the other). That leaves you with only one unknown. To solve for this unknown (and hence the other one), you need a second constraint on the system (i.e. a second equation). This constraint comes from the fact that the collision is elastic, which means that kinetic energy must also be conserved (total KE before = total KE after). Does that give you a hint as to what the second equation that escaped you is?

Oh, and by the way, it's typical to use the Roman letter p for momentum (not the Greek letter rho, which is usually used for density).

EDIT: Sorry if I messed up your attempt to guide the OP in the right direction Andrew Mason.

Wow, thanks guys for the prompt responses.

Okay so I need to use the conservation of Energy theorem?

1/2*M*V^2?

Eh?

Solve For Mass, and Plug that Back in to the Conservation of Momentum?

Anyone Care To Elaborate? I am lost.

cepheid
Staff Emeritus
Gold Member
Anyone Care To Elaborate? I am lost.
What you described in post 4 is exactly what you need to do, so I'm not sure what you want us to elaborate on. Maybe post your attempt so far and tell us which part specifically is tripping you up?

"What does it mean when the collision is "elastic"?"

The collision is elastic because both momentum and energy is conserved at the point of the collision. In an inelastic collision, some momentum and evergy is lost. For an inelastic collision, think about a bullet hitting a wooden box, which in turn makes the box slide on the floor. the bullets' energy and momentum is lost inside of the block and they become "one complete object."

Thanks Guys!

I solved the problem for a Mass of 212 kg and a Velocity of 2.18 m/s? Is that Right?

cepheid
Staff Emeritus
Gold Member
Thanks Guys!

I solved the problem for a Mass of 212 kg and a Velocity of 2.18 m/s? Is that Right?
You should really post your solution if you want us to check it.

Andrew Mason