Are these inelastic or elastic collisions?

Question

Imagine two blocks. One is moving towards the other, which is stationary. If they have magnets attached to the end of them, is this an elastic collision? I would have thought that it would be, seeing as the objects are moving afterwards. I know that it is impossible to get a fully elastic collision.
If you then imagine that plasticine was added to the sides of the blocks. When they collided, would this be an inelastic collision? I know that they would become one object and still move.
I am VERY confused with elastic and inelastic collisions!

Answer 1

If you mean the magnets repel each other and the blocks never touch, that's an elastic collision. No energy is lost because the magnetic field goes back to exactly the same state, when the blocks move apart.

When the blocks stick together, energy must be lost (deformation of Plasticine, thud of the contact = sound energy, etc.) so by definition it's inelastic.

Answer 2

Your question is very poorly stated.



Which direction are the magnets arranged? Do they attract or repel?



If they attract, the magnets cause the blocks to stick together, and travel at a common post collision speed. This would imply an inelastic collision.



If they repel and the magnetic force is the only interaction force between the blocks, it is a truly elastic collision. This is because the magnetic force between two permanent magnets is a conservative force, and kinetic energy is conserved from pre-collision to post-collision.



What is plasticine? Is it a rubber-like material or is it an adhesive? Does your teacher expect you to know this?



If it is rubber-like, then elastic forces are the force of interaction during the collision and the collision is thus elastic. This assumes no friction effects in the plasticine material. Think two bouncy balls colliding. The bouncy ball almost reaches the same height from which it was dropped upon rebound. Because such a collision may not be fully elastic, the coefficient of restitution is defined to quantify how elastic a collision is.



If it is an adhesive material (like a glue or putty), it causes a sticking between the blocks. When the blocks stick, they end up moving at a common post-collision velocity.



Definitions of elastic vs inelastic collisions:

In any collision, regardless of adjective, momentum is conserved. This is assuming that the net impulse of external forces is negligible during the collision.



In an elastic collision, kinetic energy is conserved additionally. Coefficient of restitution equals 1 in an elastic collision.



In an inelastic collision, as much kinetic energy is dissipated to (heat, sound, material damage) as possible, while still being able to maintain the conservation of momentum condition. Coefficient of restitution equals 0 is an inelastic collision.



In an "in-between" collision, coefficient of restitution is defined as the ratio of velocity difference after/velocity difference before. It is equivalently defined as the deformation impulse/the restoring impulse. This quantity is used to predict the behavior post collision of a partially elastic collision.

Answer 3

Simply put, with elastic collisions momentum is the same before and after. However, with inelastic collisions some of the momentum is lost (as result of deformation of the colliding objects).



Think about snooker balls (elastic collisions) and dollop of butter hitting the floor (inelastic).

Answer 4

Inelastic- start out separate bit end up together.