Calculating Work Done on a System

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SUMMARY

The discussion centers on calculating the work done on a system when walking up a flight of stairs. The correct expression for work done by gravitational force is -mgh, as gravity acts in the opposite direction to the motion. The participant initially misinterpreted the direction of forces, mistakenly selecting +mgh. The key takeaway is that work is determined by the force acting on the system and its direction relative to the motion.

PREREQUISITES
  • Understanding of gravitational potential energy, specifically the equation Mass * g * Height.
  • Knowledge of Newton's laws of motion, particularly the concept of work and energy.
  • Familiarity with the concept of forces acting in opposite directions.
  • Basic physics principles related to systems and forces.
NEXT STEPS
  • Study the concept of work-energy theorem in physics.
  • Learn about the role of normal force in work calculations.
  • Explore examples of work done against gravitational forces in different scenarios.
  • Investigate the implications of positive and negative work in mechanical systems.
USEFUL FOR

Students studying physics, educators teaching mechanics, and anyone interested in understanding the principles of work and energy in physical systems.

KatlynEdwards
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Homework Statement



If you walk up a flight of stairs at constant speed, gaining vertical height h, the work done on you (the system, of mass m) is:

+mgh, by the normal force of the stairs.
-mgh, by the normal force of the stairs.
+mgh, by the gravitational force of the earth.
-mgh, by the gravitational force of the earth

Homework Equations



Gravitational Potential Energy = Mass*g*Height

The Attempt at a Solution



I thought because the height was positive that the Work would therefore also be positive. And since we're gaining height that the system includes the earth, so I chose +mgh by the gravitational force of the earth... This apparently isn't right. What am I doing wrong?
 
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You have to consider which force is doing the work in addition to the direction. Gravity is the force that does work on you, and since it's direction is opposite to your motion, the work is does is negative.
 
Ah I see what I did wrong. Thanks! :)
 

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