- #1

Trying2Learn

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If a rock of 2 Newton is lifted up 3 meters, the work is NEGATIVE 6

Gravity force is down, displacement is up, the dot product is negative and we can say: "The force of gravity does work = -6"

If a rock of 2 Newton is lowered by 3 meters, the work is POSITIVE 6

Gravity force is down, displacement is down, the dot product is positive and we can say: "The force of gravity does work = 6"

I am ignoring what "I" feel, and am focused on the role played by the force (perhaps that is my problem in the following question.)

Now we turn to power. Power is the dot product of the force and the velocity.

For the following, I would rather think in terms of a bicycle going up a hill at a constant velocity.

Gravity is down, your velocity component is up. The power is negative.

Now break as you go down

Gravity is down, your velocity component is down. The power is positive.

OK, so WHO is "doing" or "exerting" the power? In both cases, the bicyclist breaks a sweat (more on the way up, of course, but there is

power exerted to hold the breaks)

With regard to POWER, WHO or WHAT is exerting it?