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When given the term of work, I think of the amount of energy being transferred from one form to another in order to do something. Personally, some keywords that help when determining work are: work being done (positive), does/doing (negative).
w is positive when the system does work , and w is negative when work is being done on the system. The same goes for heat; Q is positive when heat is added, and Q is negative when heat is taken out or absorbed by the surroundings. Both of these can be seen in the equation
ΔU= w + Q
ΔU= w + Q
In addition to the previous statement we can think about it using the analogy Lavelle gave about using an airpump on a tire. We use energy when pumping air, the energy is lost and the system which is the bike gains the energy. We exert negative work and the bike has positive work.
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