Calculating lost mechanical energy

[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!

 

[ap123]

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!

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

 

[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?

 

[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!

 

[timarli]

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!

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?

 

[haruspex]

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?

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