gravity

[person]

I just learnt the equation where you solve for gravitational energy using integration. But the equation of -GMm(1/R+1/r) states that energy is negative. How do you get negative energy?

[Tony]

Well gravity pulls you in towards the source, so it could be thought of as "negative" energy being applied.

[rdrake]

Yes, or when energy is being taken away or lost, it is often considered negative.

[person]

But, it can't have a negative potential energy because potential energy is the ability to do work. It can't have a negative ability to do work.

[Tony]

Well think of it this way... "potential energy" is the energy released by the rock as it falls towards Earth.

Now swap the two in their place. The Earth is not falling towards the rock (well... technically it is a little bit), but instead pulls it up to itself via gravity - effectivly doing the opposite of the desired effect: "Negative work" ?

It depends on your perspective and how you define "work".

[ZeroPaladn]

I was taught that work is never negative. If you were to push something in one direction (the positive direction) you are applying positive force to the object (+ times + = + in math) while pulling it away form the positive (moving in the nagtive direction) is applying negative force to it (- times - = + in math). In short, work is allways positive, regardless of direction.

[Cervantes]

No, that's wrong. Work is often negative. Consider the situation in which a giant boulder is rolling down at Indiana Jones, but this time, Indiana Jones, instead of being a coward, decides to push back on the boulder and stop it.

The boulder is moving forwards, and Jones is pushing back. He's slowing it down. That's doing negative work on the boulder. Work is (force vector) dot (displacement vector). Force is in the negative direction; displacement is in the positive direction. The dot product of those two vectors is definitely negative. (since dot product is |vector1| * |vector2| * cos(angle between them), and the angle between them is pi (or 180 degrees), and cos(180) = -1.

[MysticVegeta]

person @ Tue Jan 16, 2007 9:06 pm wrote:

How do you get negative energy?

Energy is scalar, the - sign just indicates loss of energy (exothermic) and + is when e is needed. No directions whatsoever