Energy = Force*Displacement So.............................?

Question

Energy = Force*Displacement. But if i press against a wall, there is no displacement, but i do get tired in this process. So why is energy consumed?????

Answer 1

This question made me think for about 3 minutes any way, when you press against the wall, you are enforcing the parts of your body which come to contact with the wall, as a result newtons 3rd law would sugget the wallk will apply the equal but opposite force to stop you from moving, however there is a pressure between you and the wall, for example, if you push your hand agasint the wall the atoms of your hands do get closer to the atoms of the wall, in this stage, the electrostatic repulsion will stop your hand getting into any further into the wall, you ask why energy is consumed, i tell you energy is consumed for maintaining this pressure, you are do working against the electrostatic repulsion, when you press agasint the wall, your muscle contracts, this is energy consuming is in forms of ATP, thus energy is expent even though the wall is not moved. the greater the pressure you apply to the wall the greater amount of energy is required, and when the pressure is so greater, it will break the wall and that's when the wall undergoes a molecular change, a lot more on that to follow^^

Answer 2

This has to do with the way that muscles generate forces. Inside of a muscle are millions of little heads called myosin. Each one can pull against another structure called an actin philament. It is sort of like two people holding the ends of a rope as each person pulls the rope in the two people get closer together. In this example the people are the myosin heads and rope is the actin.

When you push against a fixed object, the myosin heads are trying to pull on the actin. When they cannot move the actin, the heads reposition themselves. These myosin heads are continually trying to pull on the actin philaments but cannot exert enough force to move the wall. However, they do move back and forth a little thus each little myosin does do work (= expend energy) because there is displacement. It is only when you add up all of the millions of little displacements that you don't see any displacement at the macroscopic level.

To return to our analogy, imagine if the two people (myosin) couldn't move. They might pull the roep back and forth a few times, each time doing work, but not actually getting any closer together. Someone standing a long way away, who can see the two people but not the rope itself might ask why they are expending energy if there is not displacement. The answer is there is displacement, you just can't see it.

Answer 3

Work = Force * Distance

The unit of Work is the same as the unit of Energy....the Joule.

When you push against a wall, the force you apply is over zero distance (/ displacement) so no Work is done on the wall.
Just because you aren't doing any work on the wall does not necessarily mean that your body is not expending energy. Remember, you still need to tense up your muscles in order to apply the force in the first place, this eats up a slight amount of energy.

Answer 4

OK, the real answer, is that Energy <> Force * Displacement. It is Work (not Energy) that = Force * Displacement. By definition, because that wall was not displaced, you did no work.

On the other hand you did expend Energy...to no avail, but expend it you did. In your case, you expended electro-chemical energy through your muscles and mind. The electro-chemical process creates the energy that allows you to do work by moving things around (if only they would move).

Energy is defined as the ability or capacity to do work or cause a change. The key word here is "ability." Energy is not the work or change that energy can do, but the ability to do that work when given the chance. In your case, all that electro-chemical energy you used up never had that chance to do work because the wall, given the amount of energy you generated, was immovable.

Bottom line, energy is the potential to do work, but not the work itself.

Answer 5

I believe you mean work=force*displacement

you did no work on the system you defined (being the wall)

inside your muscles, there were many tiny instances of force and displacement

picture a car pushing on a wall, no movement and no work being done on the system defined (again, the wall), but the pistons of the engine are moving against a force and the automatic transmission torque converter is moving against a force and there is lots of work being done in these systems and lots of energy expended

Answer 6

If the object is not accelerating (or displaced), it means that you have no NET force on it, and that another, equal in magnitude and opposite in direction, force is opposing the force you are applying. This happens all the time; as you are standing on the ground, gravity certainly exerts a force upon you, but so does the ground, preventing you from sinking into it. These two forces exactly balance and thus you do not move. So, you are expending energy, you're just not doing any work.

Answer 7

The first answer is partially right, but the real reason is that not every muscle fiber in your muscles flexes at the same time when you exert yourself. They take turns flexing and then releasing, so that even if there isn't any overall movement, your muscles are moving on a small scale and thus you get tired. The same applies if you just sit there and try to hold a flex for a while; the muscle will tire out.

Answer 8

Your muscles are using chemical energy to tighten up. The muscles tend to relax, and so this is how your energy is spent, making them rigid. It's analogous to a car spinning its wheels--sure, no work is getting done in the overall motion of the car, but there is still chemical energy being used (and converted to heat, rotation, etc).

There are also physiological effects from the chemical by products of protracted muscle contraction: lactic acid is probably what you are feeling.

Answer 9

Work in Joules = force*distance.

In the case of the wall, you are applying force and the wall is applying force back. Therefore the wall undoes the work that you do.

Answer 10

This is more a biological issue than physics issue since the work you do is, as you say, zero, yet you do get tirred.

What makes you tirred is the tension of you muscles that you maintain.