# Collision and conservation of energy

1. Mar 7, 2013

### ShizukaSm

I wanted to check some concepts in collisions because I'm a little bit confused.

In a real-world collision (Where everything applies, friction and so on) conservation of momentum doesn't really apply because there is friction, which is an external force.

1)That is the only reason momentum isn't conserved, right?

Moving on, mechanical energy isn't conserved either, because there are non-conservative forces (that is, friction plus energy conversion to sound/heat)

2)And those are the only reasons mechanical energy isn't conserved, right?

2. Mar 7, 2013

### iRaid

Energy is ALWAYS conserved just in different ways. (Conservation of energy)

3. Mar 7, 2013

### ShizukaSm

Yes, sorry. I meant in the system block 1 + block 2, supposing a collision between two blocks.

4. Mar 7, 2013

### Dick

Yes, mechanical energy can be lost to heat etc. Momentum is ALWAYS conserved. The only way you can lose it is by tranferring it to something else. In the case of friction it's just transferred to the object you are rubbing against. That's Newton's third law.

5. Mar 8, 2013

### ShizukaSm

Thanks. Also, I think I should start saying 'Mechanical energy is transformed' and Momentum ins transferred' instead of 'Lost' as I was saying.