Gravity and Magnetism (Beginner Question)

1. Nov 5, 2006

Raza

Isn't Gravity and Magnetism free energy?
How does it follow the conservation of energy rule?

2. Nov 5, 2006

Danger

There is no such thing as 'free energy'. While those are both viable sources of energy, any use of them results in an increase of entropy. You can never extract as much as was available in the first place.

3. Nov 5, 2006

Raza

I know there is no such thing as free energy but where does the energy of gravity and magnetism come from?

Last edited: Nov 5, 2006
4. Nov 5, 2006

Staff: Mentor

Do you know the definition of "energy"? I assume you are thinking that an eternal force must be using energy. But a gravitational or magnetic force between two objects is not energy. Energy comes from moving up or down in a gravitational or magnetic field, and clearly that is conserved: whatever you get by moving one way you give back moving the other way.

Last edited: Nov 5, 2006
5. Nov 5, 2006

Raza

So the earth has been pulling moon close to itself for thousands of years without using any energy?

6. Nov 5, 2006

robphy

For simplicity, let's assume an ideal case: circular orbit with no frictional losses.

Although the earth does pull the moon away from a straight-line inertial path, the moon in its circular orbit doesn't get closer to the earth.

The earth pulling on the moon in a circular orbit (i.e. tangent to the radius) can be accomplished without using any additional energy because the force of pulling in this case is doing zero work.

7. Nov 5, 2006

Staff: Mentor

In that system, tidal forces sap energy from the orbit to move the moon closer to the earth, and they are most certainly conservative of energy. All of the gravitational potential energy lost can be accounted for in tidal friction (which is lost to the universe as heat).

8. Nov 5, 2006

Raza

Is this what you are saying?
F=MA
Since there is no additional energy added to make it stay in it's place, a=0, so F=M(0) making F=0?

9. Nov 5, 2006

robphy

$$\Delta K \stackrel{work-energy}{=} W_{net} \stackrel{net\ work}{=} \int \vec F_{net} \cdot d\vec r$$
Since $$\vec F_{net}$$ is always perpendicular to $$d\vec r$$ for a uniform circular orbit, the net work done is zero.

Last edited: Nov 5, 2006
10. Nov 5, 2006

Staff: Mentor

So then you don't know what energy is, do you? Where is the term for energy in that equation? Energy (work) is w=FD (for gravitational potential energy, among other things).

Once again, force and energy are not the same thing.

11. Nov 5, 2006

Raza

Can you have force without energy?

12. Nov 5, 2006

Raza

I meant to use the W=FxD but I forgot.
if F=0, then i this eqn, W=FxD, W (energy) should equal to zero. right?

13. Nov 5, 2006

Staff: Mentor

Yes, of course. They are completely different things. Can you have red without blood?

14. Nov 5, 2006

Staff: Mentor

Yes........

15. Nov 6, 2006

LURCH

Or you could consider a (slightly) more accurate description of the Moon's orbit, recognizing it as an elipse. Then it becomes appearant that the Moon does fall toward the Earth, and picks up speed as it falls, loosing potential energy while gaining kinetic. This higher speed causes the Moon to climb once again to a higher orbit, slowing down on the way up, and exchanging kinetic energy back into potential

The combination of [gain in altitude and loss of speed], and [loss of altitude and gain in speed] always brings the Moon back to where it started at the beginning of any cycle. The sum total of gains and losses is zero.

16. Nov 6, 2006

Neohaven

F is *not* equal to zero. there is a gravitational force acting on the object...

It's the distance D between the earth and the moon that stays constant (Case 1), or that moves up and down in regular circles (Case 2).

In Case 1, delta_D = 0, so, W = 0. no work done.

In Case 2, D moves up one, and down one. Say there is a gain of 1 joule when it comes closer, and a loss of 1 joule when it goes back further... we're at the same distance as before. thus, delta_D = 0, and by that, W = 0.

17. Nov 6, 2006

nekteo

so, is there any other force beside the earth gravity that pull the moon away from Earth? Such as, the Sun?

18. Nov 7, 2006

HallsofIvy

Staff Emeritus
Yes, every object in the universe has a gravitational effect on the moon- most of them negligible of course. I believe that the sun has a small but noticable effect on the moon. Jupiter, the largest of the planets, other planets, and non-solar system objects have no measurable effect on the moon.

The quick answer to the original question about "free energy" from gravity and magnetism is: gravity and magnetism are not "energy", they are forces. There is no "conservation of force" law.

19. Nov 7, 2006

Raza

Thank you all for filling my curiosity. :)