8:06
Positive & negative work | Work & Energy | Physics | Khan Academy
Khan Academy India - English
Overview
This video explains the concept of work in physics, differentiating it from its everyday meaning. It clarifies that work is done when a force causes an object to move. The core concept is that work can be positive or negative, depending on whether the force and displacement are in the same or opposite directions. Positive work occurs when the force aids motion, often causing acceleration, while negative work occurs when the force opposes motion, often causing deceleration. The video uses several examples, like pushing a cart, stopping a car, catching a ball, and gravity's effect on a thrown ball, to illustrate these principles.
How was this?
Save this permanently with flashcards, quizzes, and AI chat
Chapters
- In physics, work is done when a force causes an object to move.
- Work is calculated as the product of force and displacement (W = F * s).
- Unlike everyday usage, physics allows for work to be less than zero (negative work).
Understanding the physics definition of work is crucial because it's a fundamental concept that links force and motion, and it has specific mathematical implications (positive, negative, or zero) that differ from common language.
A lady pushing a grocery cart that moves in the same direction she is pushing it.
- Positive work is done when the force applied and the object's displacement are in the same direction.
- This typically results in an increase in the object's speed.
- The work done is a positive value.
Recognizing positive work helps predict how a force will affect an object's motion, specifically that it will likely cause it to speed up.
A lady pushing a grocery cart, where her pushing force is in the same direction as the cart's movement.
- Negative work is done when the force applied and the object's displacement are in opposite directions.
- This typically results in a decrease in the object's speed (deceleration).
- The work done is a negative value, meaning it's less than zero.
Understanding negative work explains how forces can oppose motion and cause objects to slow down, which is essential for analyzing braking, friction, or gravity acting against upward movement.
Superman pushing a car to the right to stop it, while the car is moving to the left.
- When catching a ball, the force applied by your hand is opposite to the ball's initial direction of motion.
- Your hand exerts a force to slow the ball down, meaning your force is directed against the ball's movement.
- Therefore, the work done by your hand on the ball is negative.
This example highlights how even actions that seem to involve 'doing something' can result in negative work if the force opposes the motion, leading to a decrease in speed.
The act of catching a ball, where your hand applies a force in the opposite direction of the ball's travel to stop it.
- When a ball is thrown upwards, gravity exerts a downward force.
- The ball's displacement during its upward motion is upwards.
- Since gravity's force is opposite to the ball's upward displacement, gravity does negative work on the ball.
This demonstrates how forces like gravity can do negative work, causing objects to decelerate when moving against the force's direction.
Gravity acting on a ball as it travels upwards after being thrown.
- When a ball is falling downwards, gravity exerts a downward force.
- The ball's displacement is also downwards.
- Since gravity's force and the ball's displacement are in the same direction, gravity does positive work on the ball, causing it to speed up.
This contrasts with the previous example, showing how the same force (gravity) can do positive work when its direction aligns with the object's motion, leading to acceleration.
Gravity acting on a ball as it falls downwards.
Key takeaways
- Work in physics requires both a force and a displacement in the direction of the force.
- Work can be positive, negative, or zero, depending on the relative directions of force and displacement.
- Positive work is done when force and displacement are in the same direction, often causing acceleration.
- Negative work is done when force and displacement are in opposite directions, often causing deceleration.
- The physics definition of work is distinct from its common usage and can lead to counter-intuitive results, like doing 'less than zero' work.
- Understanding the direction of force relative to displacement is key to determining the sign of work done.
Key terms
WorkForceDisplacementPositive WorkNegative WorkZero WorkAccelerationDeceleration
Test your understanding
- What are the two essential components required to perform work in physics?
- How does the direction of the applied force relative to the displacement determine if the work done is positive or negative?
- Explain why gravity does negative work on a ball being thrown upwards.
- Describe a scenario where zero work is done, even if a force is applied.
- How does the concept of positive work relate to an object's change in speed?