Elastic and inelastic collisions I don't understand it?

Question

Hello. When you want to check whether a collision is elastic or inelastic you use the principle of conservation of momentum to find the speed after the collision. I dont understand the following:
If momentum is conserved how can Energy not be conserved? I mean Energy is linked to the speeds of the paricles - if the speeds decrease then and only then would energy be lost but if they decrease momentum is not conserved. I dont get it. Whow can sometimes momentum be conserved and energy as well and sometimes only momentum. I know that momentum is always conserved but then when energy is lost where does thie enery come from? It cannot be just created?

10x for the answers but I already know everything you've said. What I dont understand is if energy is transfered to the second object so is momentum. But if momentum is conserved how can some of the energy transfered escape and be lost in that way. If energy escapes so should momentum, shouldn't it? KE = 0.5mv^2 so it is connected to the speed of the particles before and after the collision. Actually KE = momentum x 0.5v. If momentum is conserved so should the energy, right?

Answer 1

Energy is always conserved, but its form can change. In an inelastic collision, some of the kinetic energy goes into doing work on (deforming) an object. That kinetic energy is converted into heat energy.

Answer 2

Both energy and momentum are always conserved. Because they're both basically the same thing. If, say, a ball hit's a wall and stops, the energy and momentum are transferred to the wall, as well as everything the wall is connected to, meaning Earth. So Earth's rotation is affected by as much energy as was in the ball. But since the ball is so small, the energy/momentum isn't enough to actually do anything.

Answer 3

the momentum of the first object either stays the same after the collision or some energy is passed to the object that it collided with (if it's stationary). however, if the second object is moving, you have to take into account the initial momentum of the both objects. The energy before and after can be different if the object moving collides with another object but both don't move very much because some energy is lost as heat/sound etc. so if the second object is light than it might move at a higher speed and need less energy to move it.

Answer 4

If you have Java installed then this applet shows how momentum is conservered