Kinetic energy or momentum?

I'm really confused! After a collision, how would I know which formulas to use? I noticed this a few days ago, but when I use the conservation of energy to calculate the final speed of an object, the answer would be different from using conservation of momentum. I don't know why I noticed this a few days... show more I'm really confused! After a collision, how would I know which formulas to use? I noticed this a few days ago, but when I use the conservation of energy to calculate the final speed of an object, the answer would be different from using conservation of momentum.

I don't know why I noticed this a few days ago (tomorrow is my exam), I've been using kinetic energy and momentum for 3 courses now >.>

Someone please help explain when I should use momentum vs kinetic energy..
Update: Okay, here is an example: a car of mass m collides with a stationary truck of mass M the car's initial velocity is Vi. The car's final velocity is 0 find the truck's final velocity so if I do momentum p1i + p2i = p1f + p2f p1i + 0 = 0 + p2f mVi = MVf (m/M)*Vi = Vf But if I use kinetic energy K1i +... show more Okay, here is an example:
a car of mass m collides with a stationary truck of mass M
the car's initial velocity is Vi. The car's final velocity is 0
find the truck's final velocity

so if I do momentum
p1i + p2i = p1f + p2f
p1i + 0 = 0 + p2f
mVi = MVf
(m/M)*Vi = Vf

But if I use kinetic energy
K1i + K2i = K1f + K2f
K1i + 0 = 0 + K2f
0.5*m*Vi^2 = 0.5*M*Vf^2
[(m/M)*Vi^2]^0.5 = Vf or Vf = Vi*((m/M)^0.5)

^0.5 is square root

Clearly, I got a different answer....
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