What Happens to Billiards Balls in an Elastic Collision at Right Angles?

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Homework Help Overview

The problem involves two billiard balls of equal mass colliding elastically at right angles in a two-dimensional coordinate system. One ball is moving upward along the y-axis at 2.0 m/s, while the other is moving to the right along the x-axis at 3.7 m/s. The discussion centers on determining the final direction and speeds of both balls after the collision.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the conservation of momentum and kinetic energy in elastic collisions, questioning how to apply these principles in two dimensions. There are attempts to combine initial velocities and considerations of energy transfer between the balls.

Discussion Status

Some participants have offered guidance on the importance of treating momentum as a vector quantity and recognizing the need for separate momentum conservation equations for each direction. There is ongoing exploration of the implications of energy conservation and the potential outcomes for the balls' final velocities.

Contextual Notes

Participants note the complexity introduced by the two-dimensional nature of the collision and the need to consider both momentum and kinetic energy conservation without assuming a straightforward transfer of energy between the balls.

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Homework Statement


Two billiards balls of equal mass move at right angles and meet at the origin of an xy coordinate system. One is moving upward along the y-axis at 2.0 m/s, and the other is moving to the right along the x-axis with speed 3.7 m/s. After the collision( assumed elastic), the second ball is moving along the positive y axis. What is the final direction of the first ball, and what are the two speeds?

Homework Equations


it is elastic so
P1+P2=P1final+P2final
mass is constant so it cancels
V1+V2=V1final+V2final



The Attempt at a Solution


i got 5.7=V1final+V2final
 
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A couple of things that might help:

Since the collision is elastic, kinetic energy is conserved.
The problem is in two dimensions! Remember momentum is a vector quantity. (Can you just add the initial velocities to give 5.7ms^-1 as you have done?)
 
i did the KE+KE=KEf+KEf
I ended up with basically the same thing only the magnitude is squared. would the first ball come to rest and give ball 2 all its energy? that would mean that ball 2 would have a vfinal of 5.7 m/s. would that work? I am studying for a big test monday in AP Physics and this question has me beat.
 
You missed my second point. Since the collision is in two dimensions, you have two equations for conservation of linear momentum- one in the x-direction, and one in the y-direction.
Of course KE is only concerned with the magnitude of the velocity, so there is no need to resolve your KE equation into components.
 

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