# What do 'perfect elastic' and 'perfect inellastic' collision mean?

1. Apr 20, 2007

### kthouz

What does mean a perfect elastic collision and a perfect inellastic collision? Do they really exist or it is just by assuming?

2. Apr 20, 2007

### lpfr

In an elastic collision kinetic energy is conserved. In an inelastic collision the two colliding things form a new unique object (they stay glued together), and some of the kinetic energy is transformed in other forms of energy (usually heat).

Perfectly elastic collisions exist, but not with macroscopic objects. There is always some energy lost to noise and heat. But at particle level (molecules, atoms, electrons, etc.) most of the collisions are perfectly elastic.

3. Apr 20, 2007

### kthouz

conservative force.

How to find mathematically that a force is conservetive? i know by definition that a conservative force is a force in which work done is indipendent of the path followed like gravitational force.

4. Apr 20, 2007

### enricfemi

if we know the function of the force , and $$\nabla \times F=0$$

then it is conservetive

5. Apr 20, 2007

### enricfemi

but i dont know, kthouz,whether you are a college student.

if not, just remember the friction isn't

6. Apr 20, 2007

### George Jones

Staff Emeritus

$$F \left( x , y , z \right) = \frac{-y}{x^2 + y^2} \hat{x} + \frac{x}{x^2 + y^2} \hat{y}?$$

:uhh:

7. Apr 20, 2007

### lpfr

What about $$x=y=0$$?

$$\nabla\times F= 0$$ except for $$x=y=0$$

The function has a discontinuity for $$x=y=0,\,\,\, F=\infty$$
For any closed path that does not enclose the origin, any line integral of the function is zero.

Happily, in physics you don't often find functions with poles.

Anyhow, I don't think that this thread is the good one to talk about curls and discontinuities.

It is better to stick to the original question and to the level of the question. Talk about curls to someone who asks about elastic collisions is not a good idea. There are other ways to explain what is a conservative force.

8. Apr 20, 2007

### enricfemi

alright,alright, so talk about how to explain conservetive force mathematically
without curls.

9. Apr 20, 2007

### lpfr

This is a physics forum. In physics you do not need mathematics to explain anything. You explain without math and, once it has been understood, you use the math to calculate.

I can assure you that, in physics if you need math to explain something, it means that you have not well understood the subject.

A conservative force is a force whose exerted work is converted in potential energy which can be transformed back completely into mechanical work.