Physics · StudentHub Lesson
Energy & Work
How work transfers energy and the relationship between kinetic and potential energy.
What you will learn
- Define work and its formula
- Explain kinetic and potential energy
- Apply the work-energy theorem
- Describe energy conservation
- Calculate work done by a force
Watch the lesson
Work done by forces (part 1) | AP Physics | Khan Academy · Khan Academy
Watch on YouTubeTopic notes
Main Idea
Work is done when a force moves an object, transferring energy; energy can exist as kinetic (motion) or potential (stored).
Key Concepts
- Work = Force x distance x cos(angle)
- Kinetic energy depends on mass and velocity
- Potential energy depends on position, such as height
Definitions
- Work: energy transferred by a force acting over a distance
- Kinetic energy: energy of motion, KE = 1/2 m v^2
- Potential energy: stored energy due to position, PE = mgh
Processes
As an object falls, potential energy converts into kinetic energy, and total mechanical energy is conserved in the absence of friction.
Equation
W = F d cos(theta); KE = 1/2 m v^2; PE = mgh
Examples
A ball dropped from a height loses potential energy as it gains kinetic energy on the way down.
Common Mistakes
- Forgetting to include the angle in the work formula
- Confusing work with force
- Ignoring energy losses due to friction
Key concepts
Important terms
- Work
- Energy transferred when a force moves an object over a distance.
- Kinetic energy
- Energy an object has due to its motion.
- Potential energy
- Stored energy due to an object's position or condition.
Worked examples
Problem
Calculate the work done pushing a box 5 m with a force of 10 N in the direction of motion.
- 1. Use W = F * d * cos(theta)
- 2. Theta = 0 degrees so cos(0) = 1
- 3. W = 10 N * 5 m * 1
Answer: 50 Joules
Quick revision
- Work = force x distance x cos(angle)
- KE = 1/2 m v^2
- PE = mgh
- Energy is conserved in closed systems
- Work is measured in Joules
- No work is done if force is perpendicular to motion
- Energy converts between forms
Check your understanding
Question 1 · Multiple choice
What is the formula for kinetic energy?
Question 2 · Multiple choice
What unit measures work and energy?
Question 3 · True or false
Potential energy depends on an object's height above a reference point.
Question 4 · Short answer
A 2 kg object moves at 3 m/s. Find its kinetic energy.
Question 5 · Short answer
What happens to work done if force is perpendicular to displacement?
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