the definition of 'work'

[quantum quack]

maybe this squestion could be extended to another:
Does the application of force require energy?

The rock is obviously supplying a contra force to the flow of the water. Does that contra force needed to deflect the water come at a cost?

or is it just simply defined as a force?

I am thinking of Newtons law. The water will flow in a straight line unless acted on by another force. The rock provides that force but the question comes back to the above I guess; does the application of a force require energy?

[Erasmus00]

Originally Posted by quantum quack

Facinating responses guys thanks.
If we take a steel gallow frame and hang a 10 kg weight from a cable is the frame doing work?
I am assuming the term work means "to expend energy".
It's sort of like looking at a steel bar standing on it's end and asking is it expending energy to sustain it's rigidity?
I tend to think that the answer is that the energy expended is also conserved with in the bar system. [at an atomic level] that the forces within the bar generate the energy and absorb the energy in doing that generation. a cycling of energy so to speak.

it is an interesting question though, as to how to correctly assess the energy situation of our rock in a river, I must admit.

There are several "deffinitions" for work. One is the work function, which is the integral of Force*ds where s is displacement. If the force is constant, then you get the often quoted work = force*displacement. The other deffinition is that work = change in energy. If you gain energy, something has done work on you, if you lose energy, you have done work on something.
Now, if a still rock is in a moving stream, it does no work, as has been said above. The water does work on it. But if a still stream has a moving rock in it, then the rock does work on the water.
If you hang a 10kg weight from a cable, after the cable has streched a bit, no more work is done. When nothing is moving, no part of the system is gaining energy, and no part is losing energy, so no work is being done. Same goes for the rod supporting itself.
-Will

[Erasmus00]

Originally Posted by quantum quack

maybe this squestion could be extended to another:
Does the application of force require energy?

The rock is obviously supplying a contra force to the flow of the water. Does that contra force needed to deflect the water come at a cost?

or is it just simply defined as a force?

I am thinking of Newtons law. The water will flow in a straight line unless acted on by another force. The rock provides that force but the question comes back to the above I guess; does the application of a force require energy?

The easiest way to figure out if an object is losing energy to apply the work energy theorem. The energy expended is equal to the integral of F*ds, where s is displacement. If the force doesn't move the object in the direction of the force, it does no work. Consider a rock spinning on a string in a circle. The kinetic energy of the rock is .5 mv^2, and the velocity is constant, so the energy is constant. The string does no work, though it applies a force. Its energy doesn't diminish.
-Will

[Qfwfq]

It is a matter of applying the principle of relativity. From the water's point of view, the rock is doing work. Where does the energy come from? It comes from the ground, which is pushing the rock through the water. From the pov of the ground the water is loosing kinetic and potential energy because the rock adds a cause of dispersion, which adds a loss term in Bernoulli's equation.

The rigid object holding itself up does no work, that's force without motion.