Question 24

[acarlson225479]

I thought total mechanical energy is conserved in elastic collisions, but PE and KE can change? Thanks!

[admin]

[acarlson225479]

For all collisions Total mechanical energy (E) is conserved, but for completely elastic collisions KE is conserved (E = KE)? For inelastic collisions some KE is converted to heat (KE not conserved), but total mechanical energy is still conserved (E=KE +q)?

[admin]

I think we should clarify a few points. Mechanical energy depends on the position or motion of masses, not other types of energy associated with heat, light, electricity, etc. So mechanical energy into to types: potential energy (PE) and kinetic energy (KE).

It is true that for inelastic collisions, total energy is conserved but total mechanical energy is NOT conserved. Why? Sometimes the objects become deformed, and produce heat, sound etc. However, as mentioned, momentum is conserved.

[quadalpha]

"total entropy increases in any real system"

This is not a real system since there are no elastic collisions? In fact, isn't the reason there are no elastic collisions that entropy must increase with loss of kinetic energy?

[bisepourto1364]

i was a bit confused about the entropy portion as well.

are we to assume that this is a real collision?

if it was a real elastic collision --> entropy would increase according to the 2nd law of thermodynamics so entropy would NOT be conserved; KE would not be conserved either in the real world.

if it wasnt a real elastic collision --> entropy would stay constant; KE would be conserved since we're not considering friction, heat, sound, etc.

clarification, pls?