Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Power and Work
Name: Helen D.
Status: student
Age: 16
Location: N/A
Country: N/A
Date: 2001-2002


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



Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory