# Elastic collision Question

1. Oct 27, 2013

### Dan350

What would you respond on this question?
Would you correct me if necessary on my response please?
Thanks

1. A turtle and a Lion have a violent collision on a frictionless horizontal field. As you witness the collision you realize that you are in the center of mas system. After the collision you note that the lion has more kinetic energy than the turtle.Explain how can this be possible and state what you have learned about the collision the lion an the turtle from the observation.

3. For me this is an elastic collision problem. For me, the only way the lion has more kinetic energy AFTER the collison is that the turtle was heavier than the lion an was moving faster. The Lion had to be not moving before the colliision.
What I learned about the observation was that not matter the size, what matters iswhats happening on the sceneario,, in this case the turtle moving faster

2. Oct 27, 2013

### rude man

The lion's p1 = m1 v1. The turtle's p2 = m2 v2. (v2 < 0).
The collision need not be elastic. Of course, it isn't totally inelastic either (some k.e. remains).

Juggle p1 and p2 around so that 1/2 m1 v1^2 > 1/2 m2 v2^2 even though m2 << m1.

(If you're at the c.m. then the lion cannot have initial velocity = 0. Remember, the c.m. does not change at any time.)

3. Oct 27, 2013

### haruspex

As rude man says, it is not necessarily elastic. Pls post your reasoning for your conclusions.

4. Oct 28, 2013

### Dan350

So what's happening,, how can have more kinetic energy after the collision?
and, haru, what i was thinking was that the turtle had to hit the lion harder in orde to it to have more kinetic energy

5. Oct 28, 2013

### rude man

You cannot have more k.e. after the collision than before.

You have to choose v1 and v2 such that the lion has more k.e. than the turtle after the collision. See my post #2.

6. Oct 28, 2013

### haruspex

Sure, but you cannot go from there to conclude that the turtle was both heavier and moving faster. It may only be one of the two. Could it be either, or does it have to be a particular one? Or is it the momentum or KE that has to be greater?