# Conservation of momentum and conservation of mechanical energy

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?

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,.

:)

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

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

kay
Yes. Smartphone.

Thank you so much. :D

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

(:

my2cts
Why do you spell energy and momentum in capital letters

my2cts
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?
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.

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Maged Saeed
Why do you spell energy and momentum in capital letters

just to concentrate more on these keywords ,,

Mentor
The conservation of energy and momentum is theconsequence of under translation in time and position.

Say which what?

• kay and Maged Saeed