# Homework Help: Calculating lost mechanical energy

1. Mar 20, 2013

### bona0002

Hey guys,

I'm trying to complete a homework problem, but I'm not quite sure how to approach it. Here is the question: A 90.5-kg fullback running east with a speed of 4.91 m/s is tackled by a 94.7-kg opponent running north with a speed of 2.93 m/s. Determine the mechanical energy that disappears as a result of the collision.

This question actually came after the question that read: Calculate the velocity of the players immediately after the tackle.

I solved that problem with the value of the magnitude being 2.83 m/s and the value of θ = 32.0°.

Now I know that in the book, they say that E_mech = K + U (kinetic energy and potential energy), but I don't quite know how to calculate the lost mechanical energy. Any pointers of how the process would go would be appreciated!

Thanks!

2. Mar 20, 2013

### ap123

Just calculate the mechanical energy before and after, then compare them.

3. Mar 20, 2013

### bona0002

I understand how to calculate the mechanical energy after the collision, but how do I do it before? Do I calculate the mechanical energy of the runner, of the tackler, or do I add both their mechanical energies prior to the collision, and then compare?

4. Mar 20, 2013

### bona0002

Ok, just tried it by adding the two mechanical energies prior to the collision and that gave me the correct answer. Thanks for the help!

5. Mar 20, 2013

### timarli

So is there any loss in the mechanical energy? I can't think of a reason for loss :S
To me it seems like both momentum and energy is conserved?

6. Mar 20, 2013

### haruspex

It's an inelastic collision. The two masses coalesce. Energy will certainly be lost.