# Elastic vs inelastic collisions formulas

1. Jul 8, 2013

### oneplusone

When doing practice problems from my textbook, I often get confused with both of them.
MOre specifically there are usually formulas which pertain to only one of those types of collisions.

Ex: $(v_1-v_2)_i = -(v_1-v_2)_f$

Does anyone know more of these types of formulas, or tricks to solve these equations?

Thanks.

2. Jul 8, 2013

### Staff: Mentor

- you can always use momentum conservation
- in elastic collisions, you can use energy conservation in addition
- in perfectly inelastic collisions, the objects stick together afterwards, so they have the same velocity
- in the center of mass system, all those collisions are easy to study

All equations are just a result of those ideas.

3. Jul 8, 2013

### technician

Elastic collisions: KE is conserved, momentum is conserved
Inelastic collisions: KE is not conserved, momentum is conserved.
This is a reasonable starting point

4. Jul 8, 2013

### gmax137

Also, don't be confused by the word "elastic." In common usage, "elastic" is like a rubber band or a balloon, that is, something "stretchy." BUT, when we say "elastic collision" in physics, examples are hard steel ball bearings (like you see in a Newton's Cradle toy) or billiard balls. Collisions of soft stretchy things (like soft rubber balls) are likely to be "inelastic." Confusing? I thought so when I learned this stuff the first time.

5. Jul 9, 2013

### wasi-uz-zaman

elastic collision

if masses of two objects is same than in elastic collision there velocities get interchanged and if masses are not equal than you can use the formula which u have given for elastic collision.