# Homework Help: Finding ratio of impluse

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1. Jul 25, 2015

### Fuzzykatecake

• User informed about mandatory use of the homework template.
Two identical molecules A and B each of mass M travel with the same velocity towards a vertical wall at an angel 20 degrees with respect to the normal to the wall. A rebounds from the wall with the same speed while B sticks to the wall.

Determine the ratio of the implus experience by A to that by B during collision.

I know the inpluse experienced by A is
final momentum - initial momentum=
Mvcos20-(-mvcos20)

But for B, why is the impluse mv? And not 0- mvcos20

2. Jul 25, 2015

### Orodruin

Staff Emeritus
What is the velocity of B before the collision? What is the velocity after? As a result, what is the difference in B's momentum before and after?

3. Jul 25, 2015

### Fuzzykatecake

Isn't the velocity of B before collision vcos20. And velocity after is 0 since is stick to wall. Therefore the momentum is 0-mvcos20.
But the correct answer is mv which I don't understand why

4. Jul 25, 2015

### Orodruin

Staff Emeritus
No, that is the velocity component orthogonal to the wall.

5. Jul 25, 2015

### Fuzzykatecake

But why for A is vcos20, what's the difference?

6. Jul 25, 2015

### Orodruin

Staff Emeritus
A does not stick to the wall. The momentum component of A orthogonal to the wall is simply reflected while that perpendicular to it is unchanged.