# Find V2 of an elastic equation

1. Nov 25, 2016

### Angela_vaal

1. The problem statement, all variables and given/known data
A 0.038-kg pet lab mouse sits on a 0.35-kg air-track cart, as shown in (Figure 1) . The cart is at rest, as is a second cart with a mass of 0.25 kg. The lab mouse now jumps to the second cart. After the jump, the 0.35-kg cart has a speed of v1=0.86m/s.

What is the speed v2 of the mouse and the 0.25-kg cart?

2. Relevant equations
1/2m1v02=1/2m1v1,f2+1/2m2v2,f2

3. The attempt at a solution
I don't know where to start. I am assuming this is an elastic collision since the carts don't stick together after the collision. Would I use the equation listed above? I just don't know how to start

Last edited: Nov 25, 2016
2. Nov 25, 2016

### BvU

I don't know if 'not sticking together' ensures an elastic collision ....

And if the speeds beforehand were zero, then afterwards they must be zero too ?
Do you think there is conservation of kinetic energy in this case ?
What about the work mickey mouse does when kicking away the 0.35 kg block ?

Know any other conservation laws ? (in fact, one other is already enough... )

3. Nov 25, 2016

### Staff: Mentor

The mouse "sticks" to the second cart, so at least part of the scenario involves a collision that is not an elastic one.

What conservation law does your relevant equation express? Is there another quantity that's conserved that might be a better choice for the overall scenario?

Edit: Ah! BvU got there before me!

4. Nov 25, 2016

### Angela_vaal

Is the collision inelastic then?

5. Nov 25, 2016

### Angela_vaal

I'm confused.

6. Nov 25, 2016

### Staff: Mentor

It's not a single collision scenario. The mouse pushes off the first cart with what might be interpreted as an inelastic collision in reverse (separation instead of joining) and then collides with and sticks to the second cart in another inelastic collision.

The good news is that one conservation law covers the whole process from start to finish.

7. Jun 11, 2017

### ChristianBethel

And what is this conservation law?

8. Jun 11, 2017

### haruspex

You should never assume collisions are elastic without good reason. If you can solve without that assumption, do so.
As others have told you, there is another conservation law available. You must have been taught it. It applies to a system (two carts plus mouse in this case) provided there are no external forces acting on the system in the direction of interest. Since the track is frictionless, there are no external horizontal forces on that system, so the law applies in that direction.