OT: The 10th Demention

[Renegade 13]

And the force you are applying to the aforementioned large rock isn't disappearing either; it's being "eaten up" by friction between the rock and the Earth, etc. You could apply a lot of force to the rock, but if you can't overcome the coefficient of friction, all you're doing is ultimately applying all the energy you exerted on the rock into the Earth, which is holding the rock still.

[Raapys]

Yeah, but then 'what is force'? Isn't exerting force actually just transference of kinetic energy? If an object floats, completely still, in space, and another object bumps into it, isn't it true that the previously still object will start to move and the previously moving object will stop( assuming a perfect collision between like masses )?

Renegade, friction wont matter if you're lifting it straight upwards, though.

Anyway, I'll try not to drag this around in circles forever. I was just thinking that surely gravity would be both a creator and a destructor of energy.

Given a rope, a tree, a bucket of water, a gravity switch and Earth: The tree is on earth, the rope is hanging from the tree, the bucket of water is hanging from the rope and is so heavy that it'll snap the rope within one minute. The gravity switch is off.

You turn on the switch. What happens? Gravity starts pulling, exerting force as you would say, on everything. With the help of the bucket of water, gravity is actually pulling with enough energy to break the bonds in the rope.

The big question: Where's gravity getting its energy from? And even if gravity itself doesn't actually need any energy to work( i.e. spacetime ), gravity *is* exerting force on the bucket making it move. When it's moving it has kinetic energy. Since gravity made it move, that means gravity *created* energy, no?. To our knowledge, gravity doesn't weaken over time( unless given external events ). Gravity could be doing this with billions and billions of buckets all over the world, forever really. So what am I missing? How is this not an infinite energy scenario and something that breaks the law of conservation of energy?

Sorry if I'm being difficult. I understand what you guys are saying; it's along the lines of what I did learn when I actually went to school. I just can't get it all to add up.

And how is it that a photon can slow down when passing through other mass, then speed up again on its own accord? Doesn't that too go against a number of laws?

[Suicide Junkie]

You don't have to expend energy to apply a force. The spring in your ballpoint pen (the clicky kind) is always exerting force on the clicker, but that dosen't in itself mean that the pen is mightier than the sword.

Change in energy = Work = Force (dot) Distance
(Dot product of the vectors) this is the same as simple multiplication if the force and movement are in the same direction. (Negative if they're in opposite directions, and zero if they are perpendicular)

No movement means no transfer of kinetic energy regardless of the force applied.

[AngleWyrm]

Something's amiss with the Work formula. If I lift a brick straight up it takes some effort. If I lower that brick straight down, it doesn't feel like I've exerted as much effert. If I push it sideways on a slippery surface, it takes almost no effort at all.

If I carry a rock to the top of a mountain, did I store energy in some sort of battery? Potential Energy? I notice that pendulums and bouncy balls and roller coasters have enough energy in their battery to bounce all the way back up to very nearly where they were. Odd that we say it was Momentum carried it back up to where it was. If I put enough energy in a brick to lift it six feet from the sand, we have no problem saying that was just enough energy to go back down. But it looks like twice as much on the way down, doesn't it?

What if I tossed a steel marble up in a vacuum sealed metal box here on earth? Would it continue to bounce up to some specific height? And doesn't that look a lot like orbiting, viewed from an angle?

[AgentZero]

Something's amiss with the Work formula. If I lift a brick straight up it takes some effort. If I lower that brick straight down, it doesn't feel like I've exerted as much effert. If I push it sideways on a slippery surface, it takes almost no effort at all.

I only got a C in highschool physics, so I'm no expert, but the way I remember it working is this: When you lift the brick, you're working against gravity, so you personally do most of the work, so it seems hard. When you lower the brick, gravity is doing most of the work, so to you it seems easy. But either way, the same amount of work is being exerted on the brick. I think the work formula by itself is based on moving things through the air, and you have to make additional calculations for friction when you're moving an object across a surface. Or something. I was sick when we covered friction so I'm kinda fuzzy on it.

What if I tossed a steel marble up in a vacuum sealed metal box here on earth? Would it continue to bounce up to some specific height?

Things stop bouncing because every time they hit the object they're bouncing against, they transfer some of their energy into what ever they're bouncing against, so your marble would probably bounce just the same in a vacuum as it does in the atmosphere, since air friction plays a very small part in slowing the marble. Otherwise, air being thin as it is, things would bounce for a very long time!

[Raapys]

Change in energy = Work = Force (dot) Distance

But the work formula doesn't cut it. It doesn't even take into account that the force being applied could have ripped apart atomic bindings and the likes( which means that *some work* has definitely been done, regardless of what the formula says ) even if the object wasn't actually moved.

No movement means no transfer of kinetic energy regardless of the force applied.

Gravity does generate movement, and accelerates/decelerates other objects, without tiring.

But what's the difference between saying what you're saying, and saying that kinetic energy is always being transfered, it's just going elsewhere? If you're far away in space and you push something, then accoding to the work formula, work is *always* being done, because away from gravity even the smallest amount of force exerted on an object will get it to move. Agreed?

Let's not forget that kinetic energy is really *all energy*. Even the heat and sweat of your body when doing work is at base kinetic energy. For that matter, heat *is* kinetic energy; the hotter it is, the faster the particles move, the more kinetic energy they have. The work formula only takes into account work done on a visible level, it doesn't take into account work done on an atomic, or even smaller, level.

[Suicide Junkie]

It does not take any energy to apply a force.
It does take energy to accelerate an object.

You gain energy if you fall in a gravitational field (since you are moving with the force.
You lose energy if you move against the force (upwards).

If you do the integral, you will find that the escape velocity of Earth is 11.186 km/s (The kinetic energy is scaled by mass and the force of gravity is scaled by mass too, so mass cancels out)
If you are moving outwards, you lose kinetic energy and speed, but gain potential energy for being higher up and having farther to fall.
Kinetic energy is NOT all energy.

A compressed spring isn't hotter or moving any faster than a relaxed one. And that the stretched slingshot dosen't move either... and how about all that chemical potential energy in your car's fuel tank?

[Fyron]

Raapys said:
But the work formula doesn't cut it. It doesn't even take into account that the force being applied could have ripped apart atomic bindings and the likes even if the object wasn't actually moved.

Perhaps because you can't "rip apart atomic bindings" by pulling on a rope? No amount of force you can apply will overcome the strong nuclear force... Though if you are talking about inter-molecular bonds, those can certainly be broken in the process of applying (thermodynamic) work. But you seem to be thinking of mechanical work...

When you are talking about work, it makes a huge difference which type of work you are talking about. "Work" is a horribly generic word that can mean many different things.

If an object does not move, no mechanical work was done. Mechanical work is purely a derived property of force applied and distance moved. W = F * D is entirely correct, in the mechanical sense of work. Mechanical work does not take heat or other forms of energy transference into account; it only deals with force applied and distance moved.

In the thermodynamic sense of work, you can do work without moving an object. Thermodynamic work is a generalization of the mechanical concept of work; it is a quantity of energy transfered from one system to another. It includes the microscopic thermal motion of particles (aka heat), as well as macroscopic changes to the system (movement, fluid expansion, chemically charging a battery, etc.).

Lets say you decide to push on a wall, which is sturdy enough to resist all of your efforts. Was work performed? No, and yes.

In the mechanical sense, no work was done. The wall was not moved, regardless of how much force you applied. F * 0 is 0.

In the thermodynamic sense, yes, work was done. Your muscles contracted, which required burning up stored carbohydrates and such. This generated heat, which dissipated into your body, the surrounding environment, and perhaps a bit into the wall itself. The force applied on the wall by your hands pushed the wall's molecules ever so slightly inward; this is essentially imparting some kinetic energy from your hands' molecules to the wall's molecules, which in turn is rapidly converted into potential energy. The resulting electromagnetic forces pushed back, resisting any overall change to the structure or location of the wall. In the end, there winds up being no kinetic energy in the wall (when looking holistically at the wall itself). Friction at work (yet another meaning of work... bloody English ). Hopefully I didn't miss anything there, but you get the idea.

The same exact analysis applies to two people pulling on a rope (without breaking it), save that now you have heat dissipation from 2 people and tension at play. You have lots of thermodynamic work going on, but no mechanical work at all (at least, until the rope breaks).