1. What Is Work in Physics?
In physics, work has a clear meaning: work is done when a force causes an object to move. If there is no movement, no work is done — even if a force is applied.
This definition helps avoid confusion with the everyday meaning of ‘work’.
1.1. Two Conditions for Work
- A force must act on an object.
- The object must move in the direction of the force (at least partly).
If either condition is missing, no work is done.
1.2. Examples
- Lifting a bag upward — work is done.
- Pushing a box so it moves — work is done.
- Holding a heavy suitcase without moving — no work is done.
- Pushing a wall that doesn’t move — no work is done.
2. Mathematical Expression for Work
Work is calculated using:
\( W = Fd \cos \theta \)
where:
- \( W \) = work done
- \( F \) = applied force
- \( d \) = displacement
- \( \theta \) = angle between force and displacement
2.1. Understanding the Formula
If force is applied in the same direction as movement, \( \cos \theta = 1 \), so:
\( W = Fd \)
If force is perpendicular to motion, \( \cos 90° = 0 \), so work is zero.
2.2. Units of Work
The SI unit of work is the Joule (J).
1 Joule = 1 Newton of force × 1 metre of displacement.
3. Types of Work
Depending on the direction of force and motion, work can be positive, negative, or zero. These special cases will be explained in the next topic.
3.1. Quick Preview
- Positive work: force and displacement in same direction.
- Negative work: force and displacement in opposite directions.
- Zero work: no displacement, or force ⟂ displacement.
4. Simple Real-Life Examples of Work
Understanding work becomes easier with familiar activities:
4.1. Example 1 — Lifting
You lift a bucket upward with force. The bucket moves upward. Since force and displacement match, work is done.
4.2. Example 2 — Carrying a Bag Horizontally
You apply an upward force to hold the bag, but displacement is horizontal. Force is perpendicular to displacement, so work done is zero.
4.3. Example 3 — Friction Doing Negative Work
When a car slows down, friction acts opposite to the car’s motion. It does negative work by reducing kinetic energy.