How don't forces create energy?

[Baby Astronaut]

Gravity pulls on an object, which speeds up thus gaining kinetic energy.

 

Like-charged particles repel, which seems to be a method of energy production.

 

What am I missing?

Edited December 10, 2009 by Baby Astronaut
title clarification

[Sha31]

Gravity pulls on an object, which speeds up thus gaining kinetic energy.

 

Like-charged particles repel, which seems to be a method of energy production.

 

What am I missing?

 

Field potentials, i.e. potential energy.

 

It's all fields. Gravity, electric and magnetic fields. They have their potentials that drop off with distance by inverse-square law. So, gravity pulls on an object, which speeds up, thus gaining kinetic energy, but also losing potential energy, hence energy is conserved. However, this does not really explain what in the world is this potential energy and where does it come from. Its a 'property of fields' - I think of it as spherical space-density gradients around charges and masses, kind of like curvatures in space-time.

[ajb]

Think about a frictionless ball rolling back and fore in potential [math]x^{2}[/math]. I.e. Harmonic motion.

 

You see that the total energy is always conserved. Which is what Sha31 has said.

[Mr Skeptic]

Any movement perpendicular to the force takes no energy.

[swansont]

In a broader view, it's because dW = F(dot)ds

 

Forces acting in the direction of a displacement do work, and thus change the kinetic energy. Forces acting through no displacement (or perpendicular to it) do not.

[michel123456]

Any movement perpendicular to the force takes no energy.

 

I suppose you mean "uniform motion", not "any movement".

[Mr Skeptic]

I meant what I said. An orbit is not uniform motion. And changing the speed of an object requires movement not perpendicular to a force.