Positive, Negative, and Zero Work Done by a Force (With Easy Examples)
In physics, the concept of work done by a force helps us understand how energy is transferred when an object moves. Even though the word work sounds like something we do at school or in an office, in science it has a very specific meaning.
In this article, we’ll explore positive work, negative work, and zero work, along with simple real-life examples that make these ideas easy to understand.
What Is Work Done by a Force?
In physics, work is said to be done when a force causes an object to move.
Mathematically, work done depends on:
- The force applied
- The displacement of the object
- The direction of the force relative to the motion
This direction factor is what leads to positive, negative, or zero work.
1. Positive Work Done by a Force
Definition
Positive work is done when the force applied on an object acts in the same direction as the object’s motion.
Simple Explanation
If you push something and it moves forward in the same direction as your push, you are doing positive work.
Examples of Positive Work
- Pushing a box across the floor
The force you apply and the movement of the box are in the same direction. - Pulling a suitcase forward
Your pulling force helps the suitcase move forward. - Gravity pulling a falling object downward
Both gravity and motion act downward.
Why It Matters
Positive work increases the energy of an object and helps it move faster or cover distance.
2. Negative Work Done by a Force
Definition
Negative work is done when the force applied on an object acts opposite to the direction of motion.
Simple Explanation
If a force tries to stop or slow down a moving object, it is doing negative work.
Examples of Negative Work
- Friction slowing down a sliding object
Friction acts opposite to motion. - Applying brakes on a moving car
The braking force works against the car’s movement. - Gravity acting on an object moving upward
When you throw a ball upward, gravity pulls it downward.
Why It Matters
Negative work removes energy from an object and slows it down or stops it.
3. Zero Work Done by a Force
Definition
Zero work is done when:
- There is no movement, or
- The force acts perpendicular (at 90°) to the direction of motion
Simple Explanation
If a force does not cause movement in its own direction, no work is done.
Examples of Zero Work
- Pushing a wall that doesn’t move
Force is applied, but there is no displacement. - Carrying a bag while walking on a flat road
The force you apply upward is perpendicular to the horizontal motion. - Centripetal force in circular motion
The force changes direction but not speed.
Why It Matters
Zero work means no energy transfer occurs through that force.
Comparison Table
| Type of Work | Direction of Force | Example |
|---|---|---|
| Positive Work | Same as motion | Pushing a cart |
| Negative Work | Opposite to motion | Braking a car |
| Zero Work | Perpendicular or no motion | Carrying a bag |
| Type of Work | Situation / Example | Direction of Force | Direction of Motion |
|---|---|---|---|
| Positive Work | Pushing a box on a floor | Same as motion | Forward |
| Positive Work | Lifting a bucket from a well | Upward | Upward |
| Positive Work | Pulling a trolley | Same as motion | Forward |
| Positive Work | Wind moving a sailboat | Same as motion | Forward |
| Negative Work | Friction on a sliding object | Opposite to motion | Forward |
| Negative Work | Brakes applied to a moving car | Opposite to motion | Forward |
| Negative Work | Gravity on a ball thrown upward | Downward | Upward |
| Negative Work | Air resistance on a moving vehicle | Opposite to motion | Forward |
| Zero Work | Pushing a wall | Applied force | No motion |
| Zero Work | Carrying a bag on a flat road | Upward | Horizontal |
| Zero Work | Satellite in circular orbit | Perpendicular | Tangential |
| Zero Work | Holding a load while standing | Applied force | No motion |