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According to Newton's third law, the javelin exerts as much force on the athlete as the athlete exerts on the javelin. Would it be correct to say that the javelin does work on the athlete?

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    $\begingroup$ ...is the athlete moved by the force exerted by the athlete? $\endgroup$ – ACuriousMind Apr 15 '16 at 11:06
  • $\begingroup$ I suppose the athlete throws the javelin, right? $\endgroup$ – Dayman75 Apr 15 '16 at 11:08
  • $\begingroup$ athelete does not through a javelin.... $\endgroup$ – m.luqman Apr 15 '16 at 11:14
  • $\begingroup$ The net force between the athlete's hand and the javelin is zero, but the net force on the javelin itself is not zero. Thus, it accelerates and yes athletes do throw them. $\endgroup$ – honeste_vivere Apr 15 '16 at 11:23
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The javelin does work on the athlete's arm/hand equal in magnitude and opposite in sign to the work that the athlete's arm does on the javelin. Both the arm and the javelin experience a force applied through a displacement.

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I'm considering this a "Athlete throws javelin" situation.

As you correctly put it, the javelin exerts a force on the athlete. However, by the very definition of "work", you can only say the javelin does work on the athlete if the point of application of the javelin's force moves in the force's direction.

If the point of application wasn't displaced, then there is no work done by the javelin.

However, when an athlete throws a javelin, the point of application of the "javelin's force" (considering it is the athlete's hand) moves in the opposite to the force's direction, and so one might conclude that the "javelin's force" did negative work.

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  • $\begingroup$ Careful with your wording. If the athlete moves the javelin, then the javelin does negative work on the athlete. Odd phrasing, but that's the accepted language. $\endgroup$ – garyp Apr 15 '16 at 12:09
  • $\begingroup$ Yes, it's true and I'm aware of that. But isn't negative work in this case more of a mathematical formality? If work is measured in Joules, it's hard to state that the force of the javelin led to a "negative energy" transferred to the athlete. That "negative work" you state is nothing more that the consequence of the "positive" work of the athlete on the javelin. So I didn't mention it. $\endgroup$ – Dayman75 Apr 15 '16 at 12:15
  • $\begingroup$ And by "hard to state", I mean't needlessly confusing. But you're right. I shall edit my answer. $\endgroup$ – Dayman75 Apr 15 '16 at 12:16
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According to Newton's third law, the javelin exerts as much force on the athlete as the athlete exerts on the javelin.

During the throwing phase (when the athlete is in physical contact with the javelin), the athlete exerts a force on the javelin. That force acts only on the javelin. At the same time, the javelin exerts an equal and opposite force on the athlete. This force acts only on the athlete.

Would it be correct to say that the javelin does work on the athlete?

You will certainly agree that the athlete does work on the javelin.

Work done can be thought of as the change in mechanical (kinetic + potential) energy of a system. Energy is neither created or destroyed but just transformed for transferred.

For the books to balance, the javelin does negative work on the athlete.

So the answer to your question is: Yes. The javelin does work on the athlete.

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