Work Must Involve an External Force and Motion Along the Force
In order for a force to do work on a system, three conditions MUST be met:
- the force doing the work must be external to the system on which work is being done,
- the system on which work is done must move while the external force is applied (We say, "the point of application of force must move."), and
- the force doing the work must have a component in the same direction as the motion of the system or in the direction opposite the motion of the system.
Does it do Work? Self-Quiz
Scroll to the top of the page and read before attempting the quiz.
- Does the earth's gravitational force do work on a ball which is traveling along a horizontal floor? The system in this case is ONLY the ball.
- Yes.
- No.
- Does the earth's gravitational force do work on a ball which is accelerating downward? The system in this case is ONLY the ball.
- Yes.
- No.
- Does the earth's gravitational force do work on a ball which is moving upward? The system in this case is ONLY the ball.
- Yes.
- No.
- A ball is accelerating downward towards the surface of the earth. Does the earth's gravitational force do work on the system? The system in this case is the ball AND the earth.
- Yes.
- No.
- Do I do work on myself as I move up a staircase? The system in this case is ONLY me.
- Yes.
- No.
- Does Joy do work on the wall if she pushes on it for 2 hours while it remains stationary? The system in this case is ONLY the wall.
- Yes.
- No.
- Does Jayce do work on a shopping cart which is moving to the right if he exerts a force on it to the left? The system in this case is ONLY the shopping cart.
- Yes.
- No.
- Jayce pushes to the right on a cart which is moving to the left. Does Jayce do work on the system? The system in this case is the shopping cart AND Jayce.
- Yes.
- No.
- Nina pushes downward and to the right on a lawn mower. Does Nina do work on the system if the lawn mower moves to the right? The system in this case is ONLY the lawn mower.
- Yes.
- No.
- Levi is accelerating as he runs along a horizontal surface. Does the earth's gravitational force do work on the system? In this case, the system is ONLY Levi.
- Yes.
- No.