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Name: Helen D.
Status: student
Age: 16
Location: N/A
Country: N/A
Date: 2001-2002

At school we are currently doing an investigation into power using the formula Power= work/time. I know to work out the work you use the formula work= force multiplied by distance. But if someone was lifting weights would you use their force (i.e weight) or the force of the object they are lifting? Hope this makes sense.

It does not make a huge amount of sense. Weight is the support force. You seem to imply that it is the gravitational force. If weight were the gravitational force, then if you drop something, we know it is weightless, but the gravitational force is still acting on it; a contradiction.

Work = force through a displacement times cos angle between the force and displacement vectors is a useful relationship, but does not always apply. A definition would be the energy transfer due to a force. When you lift something, an object, what is the energy transfer? Your chemical potential energy transfers (via muscles) to kinetic energy of the object. But as the object increases in height, it is transferring into the gravitational field. This transfer is caused by the force applied. We know how to express the energy change in a gravitational field: mg delta y. We also know that to change the height (delta y), a force was applied. Assuming that the force you applied is constant, and during the trip, the acceleration is zero (draw a force diagram), and that you lifted the object straight up, you are able to find the force YOU applied by using Work = change in gravitational potential energy. Since delta y and the displacement x are the same values, the magnitude of the force you exert up matches the magnitude of the gravitational force acting on the object.

Power is the energy flow per unit time. It now becomes a simple matter to get the power.

Nathan A. Unterman

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