

Power and Work
Name: Helen D.
Status: student
Age: 16
Location: N/A
Country: N/A
Date: 20012002
Question:
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.
Replies:
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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Update: June 2012

