Ask A Scientist Physics Archive

name         Helen D.
status       student
age          16

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.
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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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