Conservation Of Energy

[claymore]

When I kick a football I transfer energy from me to the ball, when it stops rolling and comes to a standstill us that energy all used up and gone? If so would that contravene the law of conservation of energy?

[bhg481]

The energy has been transferred to heat from the rolling friction, the air surroounding it has been pushed into movement so has kinetic energy etc. If you could hear the ball when it was rolling that is sound energy. The energy still exists but ia a different form.

[Gizmonster]

bhg has answered your answer correctly, but just to add:
If you could roll your ball in a closed system where there was no friction, the ball would just keep rolling, as none of the kinetic energy from the ball would be lost.
This, however, would be impossibles, as you'd need to do this in a complete vacuum and the surfaces would have to be frictionless ... which is impossible.

[Peter Pedant]

yes 1,2
Heat

as a fascinating factoid - Carnot ( sadi that is,) begins with the law of conservation of energy 1841:
if it werent, you cd get energy for nothing. No one has done this for the last 6000 y so it is obviously imposs

that is how he 'proves' the first law of thermodynamics

also: any formulas , and my sales go down. - hold it - didnt Hawking say that? yup, 150 y later

[Hymie]

PP in your post you state that ‘No one has done this for the last 6000 y’ – can I take it from your statement that ancient humans (more than 6,000 years ago) did (or could have done) this?

[ClareTG0ld]

I'd also like to add that the energy doesn't need to be lost as heat, sound, etc (or pushing the air), and can also be stored in the form of potential energy. So for example if a ball came to rest partially because it had rolled up a hill, then some amount of its kinetic energy has been transferred to potential energy V = mgh, where m is the mass of the ball, h is the height gained compared to its starting position, and g= 10 is about the acceleration due to gravity.

As others have hinted, though, the *total* energy of a closed system is conserved. If it appears that a system has gained or lost energy in violation of this, then you've not accounted for some form of energy that would rebalance the equation.

[ClareTG0ld]

I'd like to mention that the law of conservation of energy applies *only* to what are termed "closed" systems (or "isolated" systems, depending on the book you read).

This means:
1. A system that, practically speaking, is the only thing in the universe: when, for example, you solve a problem like "a ball is rolling up a hill at such-and-such a speed", then only the ball and hill "exist" and nothing else does.

2. Or, a system that is perfectly closed off from everything around it: nothing can enter or leave, not even heat.

Just to give an example, then:

1. the energy of the ball in your example isn't conserved, because it's not a closed system. There's no problem with this, because of the point above.
2. If you add in the ground and the air, then you *do* get back to a closed system, and the total energy is again conserved: all the energy the ball loses can be found again in the air/ground, eg as heat.