Work and KE

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Full Member
10+ Year Member
Sep 1, 2010
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Hi guys,

I'm having a bit of difficulty with this concept. So, W=change in KE.

Let's say a hammer strikes a nail into a board. Compared to the moment before the hammer strikes the nail, the mechanical energy of the hammer after its impact will be less because the hammer has done positive work. But, if its KE is decreased from let's say 80J to 20J, wouldn't this mean W=20-80= -60J. How does the calculation yield a negative value if it has done positive work.

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When the system is doing the work, the work has a negative sign. When the work is done on the system, the sign is positive. In your example, the hammer is doing work, that work has a negative sign and KEe=KEb+W (where W<0). From the point of view of the nail, work is being done ON the nail, thus the work is positive and KEe=KEb+W with W>0.