# Homework Help: Two sphere collision -- What's the speed?

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1. Sep 29, 2016

### emily081715

1. The problem statement, all variables and given/known data
Two solid spheres hung by thin threads from a horizontal support (Figure 1) are initially in contact with each other. Sphere 1 has inertia m1 = 0.040 kg , and sphere 2 has inertia m2 = 0.10 kg. When pulled to the left and released, sphere 1 collides elastically with sphere 2. At the instant just before the collision takes place, sphere 1 has kinetic energy K1 = 0.098 J .What is the speed of sphere 1 after the collision?

2. Relevant equations
k=1/2mv^2
p=mv
3. The attempt at a solution
i used the kinetic energy and tried to solve for v. v=√0.098/(0.5)(0.04). i got an answer of 2.2m/s which is incorrect

2. Sep 29, 2016

### kuruman

What you are given is the KE before the collision. When you used KE = (1/2)m1 v2 to find v, you found the speed of m1 before the collision, not after. You need to relate the speed before the collision to the speed after the collision. How can you do that?

3. Sep 29, 2016

### emily081715

Ki=kf ?

4. Sep 29, 2016

### kuruman

That's one equation. There is another that holds for all collisions.

5. Sep 29, 2016

### emily081715

M1v1+m2v2=m1v1f+m2v2f

Except I don't have either speed after the collision. Would I need to use both equations and Isolate one variable and sub it back into the other equation to to solve for one?

6. Sep 29, 2016

### kuruman

That's exactly what you need to do.

7. Sep 29, 2016

### Simon Bridge

What was your reasoning?
Have you never seen a Newton's Cradle?
[edit: never mind - you are being helped already]

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