# Homework Help: A quick question

1. Jul 1, 2008

### prdoring

First off i need to make the diclaimer that this is not for any homework or any assignment of any type... i just need to know the answer, and I am not a psyics guy... so please help me out here

A ball weighing .5 lbs travels a vertical distance 3 feet downward before changing direction to travel on a horizontal path. Shortly thereafter, it strikes another ball (of same weight) which then travels some distance before striking another identical ball. What are the velocities of the three balls after the second collision? Is there a third collision? You may neglect friction and consider the balls as particles. The coefficient of restitution between all three balls is .85.

2. Jul 1, 2008

### Staff: Mentor

Sure looks like homework to me, and definitely doesn't belong in the general forums. Thread moved to Homework Help, Intro Physics.

prdoring, we do not do your work for you here on the PF. Where did this question come from? If you're not a physics guy, why do you need to figure this out? Who is telling you that "you may neglect friction and consider the balls as particles"? Sounds pretty didactic to me.

BTW, you did not give enough information to solve the problem anyway.

3. Jul 1, 2008

### prdoring

not sure what the question is about... to tell the truth its part of this game me and my friend are playing... sortof a pseudo scavenger hunt... we send eachother rediculous questions... and we have to use all available resources to find the answers...

4. Jul 1, 2008

### Staff: Mentor

Fair enough. But the question is too much like homework/coursework for us to let it sit in the general technical forums, and Homework Help rules still have to apply. It's too easy for students to use the PF for cheating, without the Rules that we have in place for the Homework Help forums.

So all I can do is offer you a couple hints and pointers to information that you can use to solve the question yourself. First, you would need to know whether the ball was released from rest, or had some initial velocity (that's the main thing that's missing in the question). Second, you would use the kinematic equations of motion (the ones for constant acceleration due to gravity) to tell you the velocity of the ball right when it got to the floor and hit something at an angle to change to sideways velocity:

http://en.wikipedia.org/wiki/Kinematics

Then you would use something like conservation of momentum to get to the final velocities of the balls after the collisions

http://en.wikipedia.org/wiki/Conservation_of_momentum#Conservation_of_momentum

5. Jul 2, 2008

### prdoring

still completly lost man... atleast throw some formulas my way

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