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Haocheng Zhang 2A
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Joined: Fri Sep 29, 2017 7:04 am


Postby Haocheng Zhang 2A » Sun Jan 21, 2018 3:28 pm

Air in a bicycle pump is compressed by pushing in the handle. The solution says the work with respect to the air in the pump is positive. But if that's true, then in order to do positive work, doesn't the air in the pump have to expand and release energy? I am confused.

Joshua Hughes 1L
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Re: 8.3

Postby Joshua Hughes 1L » Sun Jan 21, 2018 3:49 pm

According to the textbook
"When energy leaves the system as work, the internal energy of the system decreases and w is negative"(in other words when the system is doing work). if the air expanded and release energy the system is doing work and releasing energy (thus the negative sign)
When work is done on the system (on the air by the pump) then I guess it would be positive because energy is going into the system if that makes sense

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Re: 8.3

Postby Jessica_Singh_1J » Sun Jan 21, 2018 4:43 pm

Work is positive because it is being done ON the system (energy is added), whereas if the pump expanded (energy is expended) work would be negative because it is being done BY the system.

Nicole Anisgard Parra 2H
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Joined: Sat Jul 22, 2017 3:01 am

Re: 8.3

Postby Nicole Anisgard Parra 2H » Mon Jan 22, 2018 8:53 pm

When you push in the piston on the bicycle pump, you are doing work on the system, and therefore, adding more energy to the internal system. This, in turn, indicates that the work would be a positive value, as you are adding energy.

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