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Energy is expended when a person exerts force on a object.

1) From the equation Work Done = Force x Distance, if a force is exerted, but no distance moved, then no work is done. Hence, the statement is false?

2) However, when a person exerts a force, isn't he expending chemical potential energy? In other words, there is an energy change from chemical to thermal. Hence, the statement is true?

I am not sure which of the above is right.

David Richerby
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Murtuza Vadharia
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4 Answers4

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1.) As you assumed, since the object does not move any distance, then by the definition of work, no work is done and thus the statement is true - no matter how much force is exerted, if there is no displacement, no work is performed.

2.) If a person exerts a large force on an object and the object does not move, there is no external work done. The muscles in the person applying the force are contracting isometrically yet no external work is performed by the shortening or lengthening of the muscle fibers. However, a person will be fatigued which implies that there is in fact an energy expenditure. In muscles, this potential energy is stored in the series elastic component of the muscle. Internal work is performed by the contraction and releasing of the muscle fibers much like a spring. Again using the definition of work, we can say that this statement is false.

In physics, work is generally referred to external work.

  • Apparently this answer warranted a downvote? To that person, what is at fault with this answer so I can necessarily correct any errors. Feedback without any comments isn't necessarily feedback at all. –  Nov 04 '14 at 14:00
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The first statement is an idealisation when no environmental effects are taken into account (nor heat).

The second is more realistic and takes into account environmental effects in a thermodynamic setting.

So for an idealisation it would be true while in a thermodynamic setting it would be false.

Now, one basic issue here is the definition of Work, which is not equal to any energy exchange in a system but only to specific energy changes related to given applied force (and only to that).

Nikos M.
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1) From the equation Work Done = Force x Distance, if a force is exerted, but no distance moved, then no work is done. Hence, the statement is false?

2) However, when a person exerts a force, is he not expending chemical potential energy? In other words, there is an energy change from chemical to thermal. Hence, the statement is true?

I am not sure which of the above is right

They are both right, even if this is paradoxical. If a force is applied to an object and this does not move no 'mechanical work' is done according to newtonian physics even though energy is spent. This happens because that energy spent is turned into other form of energy but does not become kinetic energy and there is no dislocation.

If you want to understand the historical reasons why this peculiar definition was chosen, you can find detailed explanations a this links : work follow the links there, and wasted energy

bobie
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    This happens because that energy does not become kinetic energy That's not the relevant criterion. For example, I could lift a weight at constant velocity, in which case I am doing work, but there is no change in its KE. –  Nov 03 '14 at 16:51
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    @BenCrowell, " I could lift a weight at constant velocity,..." you are wrong, you are confusing constant (v) KE with net KE. If a body is travelling in vacuum at constant v, then KE is constant, if you are moving against negative work you must continuously feed motion producing new energy (=> KE => v) to keep velocity constant. If you have any further opinions, please write an answer at my quoted links, where I can refute them with a thorough analysis – bobie Nov 04 '14 at 06:09
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    @BenCrowell: Gravity is acceleration (at the surface of the source), which means that in order to lift something at constant velocity as measured from this surface, you actually must accelerate it. – bright magus Nov 04 '14 at 07:36
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Not necessary.

Example: normal force of physics textbook resting on desk.

Joshua
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