# Sphere collision physics help

1. Jul 15, 2009

### Nguyenfa.t

1. The problem statement, all variables and given/known data

Example 6: Two spheres
shown in the figure below. They both hang under the influence of
gravity on 2.0 m long strings and start at rest. The 2.0 kg sphere
starts at an angle  to the vertical such that
2.0 kg sphere is released. (
assume that g = 10.0
1. What is the speed of the 2.0 kg sphere just before it
collides with the 4.0 kg sphere
2. Assuming a perfectly elastic collision
the 2.0 kg sphere immediately after the collision
3. After the collision, what is the maximum possible angle
achieved by the 2.0 kg sphere
swings back up)?
4. If both spheres had a mass of 2.0 kg, what would the speed
of the incoming sphere have been after the perfectly
elastic collision (

I just need help with number 4 I did the rest. Thanks guys

2. Relevant equations

3. The attempt at a solution

2. Jul 15, 2009

### LowlyPion

Re: Help

3. Jul 15, 2009

### Nguyenfa.t

Re: Help

i have attached the problem and the work that i have done. if someone could show me how to do number 4 i would really appreciate it. the work and the solution would be great.

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4. Jul 15, 2009

### Nguyenfa.t

Re: Help

5. Jul 15, 2009

### Nguyenfa.t

Re: Help

can somebody please do number 4. I would really appreciate it.

6. Jul 15, 2009

### Nguyenfa.t

Re: Help

can somebody please solve number 4 for me by midnight. thanks

7. Jul 16, 2009

### majin_andrew

8. Jul 16, 2009

### Nguyenfa.t

Re: Help

im in pacific time zone.

is there any way you can work it out for me please? ive been trying to do this problem for a while now and cant seem to figure it out

9. Jul 16, 2009

### Nguyenfa.t

Re: Help

after using that link i got the answer of 2 m/s is that correct?

10. Jul 16, 2009

### queenofbabes

Re: Help

The question asks about two equal masses in a perfectly elastic collision, so the answer should be zero. It's the only way to conserve both energy and momentum.

11. Jul 16, 2009

### Nguyenfa.t

Re: Help

is there any way you can show the work for that please?

12. Jul 16, 2009

### Nguyenfa.t

Re: Help

but the masses arent equal one is 2 kg while the other is 4kg

13. Jul 16, 2009

### queenofbabes

Re: Help

"4. If both spheres had a mass of 2.0 kg..."

If you could do 1-3, how can you not do 4? It's the same thing with a different value of mass!

14. Jul 16, 2009

### Nguyenfa.t

Re: Help

oh my bad misread the problem. i did 1-3 in class and have no idea how to do 4

15. Jul 16, 2009

### Nguyenfa.t

Re: Help

can you please just show me how its done. thanks

16. Jul 16, 2009

### Nguyenfa.t

Re: Help

i use the mv=mv+mv formula right then what?

17. Jul 16, 2009

### queenofbabes

Re: Help

That was to conserve momentum. Now conserve energy, KE of sphere before collision = KE of both spheres after collision. It's exactly the same as question 2.

18. Jul 16, 2009

### Nguyenfa.t

Re: Help

i don't see how you get 0 can you explain please by showing the work. it would be greatly appreciated.

19. Jul 16, 2009

### queenofbabes

Re: Help

Conservation of momentum:
$$m_1u = m_1 v_1 + m_2 v_2$$

Conservation of energy:

$$\frac{1}{2}m_1 u^2 = \frac{1}{2}m_1 v_1^2 + \frac{1}{2}m_2 v_2^2$$

You know m1, m2, and you can work out u. Solve for v1.

20. Jul 16, 2009

### LowlyPion

Re: Help

You will note that to conserve both energy and momentum after impact with twice the mass involved, something has to give.
You at once have Va = Va' + Vb' and Va2 = Va'2 + Vb'2.
Since Vb' must be non-zero, there is only one condition that can eliminate the extra middle term 2Va'Vb', namely that Va' must be 0.

Va'2 + 2Va'Vb' + Vb'2 = Va'2 + Vb'2