Question

Regarding inelastic colisions and convervation of momentum and kinetic energy.

I do not understand how can there be conservation of momentum when there is no conservation of kinetic energy.

Ec = 1/2mv^2
p = mv

If there is no conservation of kinetic energy, that means either the mass or the velocity of the particles changed, and that would imply a change in momentum as well.

What am I missing?

Answer 1

Conservation of momentum is implied directly from newtons 2nd and 3rd laws of motion.

F = m (dp/dt)

If a net force acts on an object then its momentum will change, if no force acts on the object then its momentum does not change.

Now think of a system of isolated particles that collide with NO external forces acting on them. The particles will exert the same force on each other over the same time interval when they collide due to newtons third law. Meaning that the net change of momentum will cancel out. The individual momentum of the particles may change, but the total momentum of system does not. So, momentum must be conserved in an isolated system.

Kinetic energy can be transformed in collisions to different types such as sound, heat, etc and thus kinetic energy does not need to be conserved.

Hope this helps :)

Answer 2

Cheers for that, after reading your answer and watching walter lewin's class on the subject I got it.