Question about kinetic energy...?

Question

I need to calculate the total initial and final kinetic energy for an inelastic and perfectly elastic collision. The problem I am having is understanding which variables to use to solve the kinetic energy equations.

The first part of the problem I have the average intial velocity of a large glider (V1): 53.7cm/s
The second part of the problem I have the average velocity of the large glider after the collision (V2): 74.6 cm/s
And the third part of the problem I have the average velocity of the small glider after the collision (V3): 20.4

Any help with this would be greatly appreciated, since the physics department at my college is horrible and I have no access to a tutor. Thanks!

Answer 1

There is information missing, or are you expected to give your results in terms of m1 and m2

You need to know the mass of each glider. It would help to have the initial velocity of the small glider and the direction headings for each glider. Just on the surface of it it looks like the smaller craft would have to be only marginally smaller or travelling much faster than the larger glider when it struck the larger glider from behind. The problem can be more complex if the smaller glider hits from behind at an angle, but this information is not given and since this increases more the initial velocity of the smaller glider, and there is a limit to a gliders speed, being struck directly behind seems a reasonable assumption

KE=(1/2) mv^2 and in a perfectly elastic collision, KE is conserved, so an answer could be deduced in terms of m1 and m2. Since the larger glider obviously gain KE, the smaller glider must lose an equal amount, and must have been traveling faster than 53.7 cm/s. Therefore, you can state the velocity of the smaller glider in terms of mass. All you need to do is the Algebra.

Answer 2

You ALWAYS have conservation of momentum, regardless of the type of collision. This allows you to express the initial velocity of the small glider as a function of the masses of the two gliders and the known velocities.

You can then compare the kinetic energies to determine whether the collision was actually elastic or not.