# Problem in Perfectly Elastic Collision w/o given mass

1. Jul 23, 2009

### naiian02

what formula/s should i use in this problem:

2 identical bodies move forward each other. One has a velocity of 4 m/s and the other has a velocity of 6 m/s assuming a perfeclty elastic collision. Find their velocities after collision

pls help me p. elastic col is not included in my book XD

2. Jul 23, 2009

### Staff: Mentor

What's conserved?

Kind of hard to believe that a book that doesn't cover elastic collisions would have a problem involving elastic collisions. What book are you using?

3. Jul 23, 2009

### RoyalCat

Ooh, these questions are really beautiful.

To make the math MUCH MUCH more simple, move your observer to the center of mass frame.
Try and come up with a SIMPLE expression dealing with the relative velocities of the two objects before and after colliding. You'll be surprised.

4. Jul 23, 2009

### JazzFusion

naaiin: Look in your text for the definition of an eleastic collision. Check the chapter with this problem, and look in the book's index.

Saying a collision is 'elastic' actually gives you a lot of information about it.

5. Jul 24, 2009

### cyy91

m1u1 + m2u2 = m1v1 + m2v2

its wad i've been taught in my school...
2 identical bodies means both object is the same...
assuming both objects have the same mass...
in a perfect elastic collision, both objects exchange velocity...
so the 1 that has 4ms-1 now has -6ms-1 while the 2nd 1 will be having 4ms-1
i'm not 100% sure of this though...do correct mii if any1 sees flaw in my statement...

6. Jul 24, 2009

### RoyalCat

It's true. Ignoring the time of the collision, it is as if they went "through" each-other. Unless you distinguished the two by coloring them, or some other method, you wouldn't know if there was a collision or if each just retained its velocity. :)

Try and prove this with considerations of energy, though. That's the key point of the exercise.