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Positive, negative and zero work
Beginner
Work is negative if the force opposes motion and zero if perpendicular.
Work is positive when force has a component along the motion (energy added), negative when it opposes it (energy removed), and zero when perpendicular. Friction does negative work on a sliding body.
Gravity does −mgh on a rising ball, +mgh on a falling one.
Negative work isn't "less than nothing", it's energy taken from the body.
In more detail
Work is positive when the force has a component in the direction of motion, negative when it opposes motion, and zero when the force is perpendicular to motion.
Real-world uses
- Friction does negative work on a sliding box, removing its kinetic energy.
- Gravity does positive work on a falling object, increasing its kinetic energy.
- A car's brakes do negative work on it, removing kinetic energy, while the engine does positive work to speed it up.
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Frequently asked questions
Can work be negative?
Yes. Negative work means the force removes energy from the object.
When is work zero?
When the force is perpendicular to displacement, or when there is no displacement.
Can a force do work without moving the object?
No. Work needs a displacement along the force. A wall pushed hard does no work on the wall because it does not move.
Related topics in Work, Energy & Power
Work done by a constant force · Units of work and energy · Work from a force–displacement graph · Work done by a variable force · Work done by a spring