Calculating Collision Speed: Momentum or Kinetic Energy?

[2sin54]

Hi. Sorry if this looks too beginner-ish question.

Lets say we have two objects moving towards each other. One is heavier than another and we know it's speed. We want to find out how fast should lighter object move so that after the collision both objects would stop.

Question is, what should we use to calculate the speed of the lighter body, kinetic energy's equation or momentum's? Those two give slightly different answers.

Thank you.

 

[Doc Al]

If you want the objects to stop after they collide, the total momentum must be zero. Kinetic energy will not be conserved, since the collision must be inelastic. (Momentum is always conserved.)

 

[chrisbaird]

Momentum is always conserved and total energy is always conserved, so you can use either one in principle. But total energy includes kinetic energy, heat energy lost due to friction, potential energy, etc. so it is easier in many problems to use conservation of momentum. Note that if there are enough unknowns, you may need to use both conservation of energy and momentum.