OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Recall and apply: kinetic energy (J) = ½ × mass (kg) × (speed (m/s))²
Full specification wording
Motion: recall and apply: kinetic energy (J) = ½ × mass (kg) × (speed (m/s))²
Kinetic energy is the energy a moving object stores because of its motion, measured in joules (J).
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 4 of 14
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Kinetic energy is the energy a moving object stores because of its motion, measured in joules (J). The equation is Eₖ = ½ × m × v², where m is mass in kilograms and v is speed in metres per second. Because speed is squared, doubling the speed multiplies kinetic energy by four, while doubling the mass only doubles it. Worked example: a 2 kg trolley moving at 3 m/s has Eₖ = ½ × 2 × 3² = ½ × 2 × 9 = 9 J. Convert grams to kilograms by dividing by 1000, and convert km/h to m/s by dividing by 3.6 before squaring. Always square the speed first, then multiply by mass, then halve.
What earns the marks
- Squares the speed before multiplying by mass and one half.
- Converts mass to kilograms and speed to metres per second before substitution.
- States the unit of kinetic energy as the joule (J).
- Recognises that doubling speed quadruples kinetic energy because speed is squared.
Common mistakes and how to put them right
Squares the mass instead of the speed.
Put it right: Square only the speed term.
Forgets to halve the product.
Put it right: Apply the factor of ½ after multiplying mass by speed squared.
Uses grams or km/h directly in the equation.
Put it right: Convert to kg and m/s first.
Doubles kinetic energy when speed doubles.
Put it right: Speed squared means a factor of four.
Exam tips
- Write the equation, then list m and v with units before substituting.
- Square the speed as a separate step to avoid order-of-operations slips.
- Check the answer is in joules and is positive.
- Use the factor reasoning to answer ratio questions quickly without full calculation.
Try the lesson
The start of the Motion: recall and apply: kinetic energy (J) = ½ × mass (kg) × (speed (m/s))² lesson, made from checked teaching notes for this specification point.
Have a go first
A 2 kg model car moves at a steady speed of 3 m/s. Calculate its kinetic energy.
Listen
Kinetic energy is the energy a moving object stores because of its motion, measured in joules (J). The equation is Eₖ = ½ × m × v², where m is mass in kilograms and v is speed in metres per second. Because speed is squared, doubling the speed multiplies kinetic energy by four, while doubling the mass only doubles it. Worked example: a 2 kg trolley moving at 3 m/s has Eₖ = ½ × 2 × 3² = ½ × 2 × 9 = 9 J. Convert grams to kilograms by dividing by 1000, and convert km/h to m/s by dividing by 3.6 before squaring. Always square the speed first, then multiply by mass, then halve. A strong answer earns marks for these points. Squares the speed before multiplying by mass and one half. Converts mass to kilograms and speed to metres per second before substitution. States the unit of kinetic energy as the joule (J). Recognises that doubling speed quadruples kinetic energy because speed is squared. Exam tip: Write the equation, then list m and v with units before substituting.
See it
Put it in order
Put these steps in the order they happen.
- 1. Square the speed
- 2. Convert mass to kilograms and speed to metres per second
- 3. Write the equation: Eₖ = ½ × m × v²
- 4. State the answer in joules (J)
- 5. Halve the product
- 6. Multiply by mass
Key words
Say what each word means before you turn the card.
Kinetic energy
Try answering aloud or in your head before turning the card.
Speed squared
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Unit conversion before substitution
Try answering aloud or in your head before turning the card.
Rearranging and applying a physics equation
Try answering aloud or in your head before turning the card.
See every definition
- Kinetic energy
- The energy a moving object stores because of its motion, measured in joules (J).
- Speed squared
- Because speed is squared, doubling the speed multiplies kinetic energy by four, while doubling the mass only doubles it.
- Unit conversion before substitution
- Convert grams to kilograms by dividing by 1000, and convert km/h to m/s by dividing by 3.6 before squaring.
- Rearranging and applying a physics equation
- Always square the speed first, then multiply by mass, then halve.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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