# Homework Help: Collisions: any ideas?

1. May 18, 2006

### Supernerd2004

I was assigned this problem from my physics teacher, but it doesnt really make sense. I would ask the teacher, but I dont go back to class for a few days. At this point, I just need help trying to understand what the problem is asking for. Thanks very much in advance for any help.

Find the height that mass #2 will be raised if m1 = 3lbs and m2 = 5lbs, and the length of the rod is 3 feet, if the collision is: a)elastic or b) inelastic.

Answers a) V4 = 10.39, h2 = 1.69
b)V3 = 5.19 h1 = .42

http://server5.pictiger.com/img/308709/picture-hosting/physics-problem-2.php [Broken]

Dan

Last edited by a moderator: May 2, 2017
2. May 18, 2006

### Hootenanny

Staff Emeritus
Is that the question as given? If not could you quote is exactly as printed please.

~H

Last edited by a moderator: May 2, 2017
3. May 18, 2006

### Supernerd2004

Yup, that is the exact question. This could be why im so confused. Thanks for the quick reply.

Dan

4. May 18, 2006

### Hootenanny

Staff Emeritus
Ahh, it seems to be that M1 and its rod pivots about the joint between the two rods and is released from rest, it would then collide with M2 which is also pivoted. Find the height than M2 rises to.

~H

5. May 18, 2006

### GregA

hmm...If I fill in the gaps and assume that both rods are free to rotate about the centre it seems like a case of "If I let go of the horizontal rod how fast will the bottom ball be hit by the horizontal ball and how high will it go"...does this seem feasable, can you think of any ways to solve it? (perhaps you should find out firstly how much PE m1 has)

*edit* sorry hoot didn't see your response whilst I was typing this

6. May 18, 2006

### Hootenanny

Staff Emeritus
No problem Greg, the more the merrier!

~H

7. May 18, 2006

### Supernerd2004

Oh, I see what you guys are saying. So the ball on the end rotates down and slams against the other ball. I was sitting there trying to figure out where collisions came in to play and it originally wasnâ€™t making sense. Ill do some work on the problem and update the post. Thanks so much!

Dan

8. May 18, 2006

### Supernerd2004

That was totally the right approach for the problem. I found the PE at the top of the swing, and then converted that PE to KE at the point of the collision. After calculating the collision velocities, I reconverted back into energy and found the height at which it would swing. Thanks for all the help, you guys rock!

Dan