# Need some help

I just did a lab on collisions. We used a something like an air hockey table but different. Anyways i have to determine whether or not momentum and kinetic energy are conserved.

If there are no unknowns and you have the following information how would you determine if kinetic energy and momentum are conserved.

Mass 1 = 0.541 kg, Mass 2 = 0.541 kg, Vi 1 = 0.33 m/s, Vi 2 = 0, Vf 1 = 0, Vf 2 = 0.3 m/s.

All i need to know is if i should solve for Ekbefore first and then Ekafter or should i do Ekbefore=Ekafter that way all the masses and 1/2's cancel out. So which way is right.

I am not entirely sure about what you are trying to do (although my teacher will probably think i should be), but this looks like a fairly simple one to me. With simple question, the best thing to do usually is just starting to solve whatever you can. That should take less time than posting anything here.

But i have no unknowns. I have to determine LS=RS sort of. But when i solve the left separately and the right side i get one answer but when i make both equations equal each other i get a different answer. Which is right?

daniel_i_l
Gold Member
If this is a headon collision (no angle) then simply find the initial and final KE of each mass. If the total initial KE is equal to the total final KE then the KE is conserved. Same for momentum.

so i do this.

Ekbefore=Ekafter
1/2m(1)Vi(1)^2 + 1/2m(2)Vi(2)^2 = 1/2m(1)Vf(1)^2 + 1/2m(2)Vf(2)^2
0.33^2=0.3^2
0.1089=0.09

There LS=RS sort of. Is that how you do it.

daniel_i_l
Gold Member
Thats how you do it. That would mean that the KE isn't conserved. This is because that some of KE turns into heat energy during the collision and due to friction.

dlgoff