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When a shotgun fires, the expanding gases in the barrel push the shell out. According to Newton's third law, the shell exerts as much force on the gases as the gases exert on the shell. Would it be correct to say that the shell does work on the gases?

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  • $\begingroup$ Please change your title to be more descriptive. $\endgroup$ – garyp Aug 10 '16 at 13:40
  • $\begingroup$ What do you mean? $\endgroup$ – Lady Lourdes Murillo Aug 10 '16 at 13:42
  • $\begingroup$ It means the title needs to be related to the question so that if someone sees just the title they know what the question is about, etc. This is also known as common sense. $\endgroup$ – Hasan Aug 10 '16 at 13:45
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    $\begingroup$ Try "When a gun is fired, does the shell do work on the gas?" $\endgroup$ – mmesser314 Aug 10 '16 at 13:45
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    $\begingroup$ Does the shell move? If it doesn't, how can it perform work? $\endgroup$ – CuriousOne Aug 10 '16 at 13:49
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In this scenario the shell would be doing negative work on the gases as the force the shell is exerting on the gases is in the opposite direction of the system's movement.

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  • $\begingroup$ In a frame of reference where the shell is momentarily motionless, the shell pushes on the gas, and the gas moves away from the shell. Likewise the gas pushes on the shell, and the shell moves away from the gas. They do positive work on each other. $\endgroup$ – mmesser314 Aug 10 '16 at 13:44
  • $\begingroup$ The shell is pushing on the gas but the gas's inertia keeps it moving despite this force. Where is the backwards motion that would allow for positive work to be done on the gas? $\endgroup$ – Ulthran Aug 10 '16 at 13:47
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    $\begingroup$ @mmesser314 the frame of reference in which the shell is motionless is accelerating (non-inertial) which makes it look like work is done on the gas. In fact, the internal energy of the gas decreases (in the gun frame of reference). I am afraid your comment is likely to confuse rather than illuminate. $\endgroup$ – Floris Aug 10 '16 at 13:48
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    $\begingroup$ @Floris Yes, you are right. In that frame, the gun is moving away at high speed, so the question becomes what work is done there. It is the same situation as in the lab frame, but more confusing. $\endgroup$ – mmesser314 Aug 10 '16 at 13:56
  • $\begingroup$ Another way to see it is that total energy of shell increases while that of gas decreases, so gas is doing work on shell. $\endgroup$ – Deep Aug 11 '16 at 7:28
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Would it be correct to say that the shell does work on the gases?

No. Work is defined as the product of the force exerted on an object times the distance the force actually moves the object. In this case the barrel does not move, and therefore it performs no work.

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