Conservation of momentum and conservation of mechanical energy

1. Jan 3, 2015

kay

What are the conditions necessary for applying the conservation of mechanical energy or the conservation of momentum? Can we apply these anywhere or do we require specific conditions which have to be fulfilled for applying them?

2. Jan 3, 2015

Maged Saeed

$$W_{external}=\triangle E$$

So if there is no external work , the ENERGY is conserved , otherwise the change in energy is equal to the external work.

The same thing has to be with momentum ,

$$F_{external}=\frac{\triangle L}{t}$$

If there is no external force the MOMENTUM is conserved too,.

:)

3. Jan 3, 2015

kay

I can't understand the expressions. They appear to be in the code form and not the actual formula types. :|

4. Jan 3, 2015

Maged Saeed

How is that ??! Are you logging in from your smartphone or tablet ?

Any way , the first formulas is

W= [Delta] E

F= ([Delta] L) / ([Delta] T) ,,.

5. Jan 3, 2015

kay

Yes. Smartphone.

Thank you so much. :D

6. Jan 3, 2015

Maged Saeed

Your smartphone may not supporting latex , the language used to write the mathematical and physical expressions and equations ,,

(:

7. Jan 3, 2015

my2cts

Why do you spell energy and momentum in capital letters

8. Jan 3, 2015

my2cts

The conservation of energy and momentum is the consequence of invariance of the physical problem at hand under translation in time and position.
For example in a crystal only discrete translations leave the physics unchanged. As a consequence momentum is not fully conserved.

Last edited: Jan 3, 2015
9. Jan 3, 2015

Maged Saeed

just to concentrate more on these keywords ,,

10. Jan 3, 2015

Staff: Mentor

Say which what?