What mistakes do students make with Kinetic and potential energies?

Forgetting to square the speed. Square the speed before multiplying by mass and 1/2. Using mass in grams instead of kilograms. Convert mass to kg by dividing by 1000 if necessary. Confusing kinetic energy with momentum. Check the equation and units; momentum has units kg m s⁻¹, energy has units J.

What are the exam tips for Kinetic and potential energies in Physics OCR A-Level?

In multiple-choice questions, check whether the options involve squaring the speed; eliminate any that do not reflect the v² dependence. If a question asks for the effect of changing speed, remember that kinetic energy is proportional to v², so a 3× increase in speed gives a 9× increase in kinetic energy. Always write down the equation and substitute values with units to avoid errors; this also helps you check the unit of the answer.

OCR · A-Level · Physics

Course version: H556 · Version from Sept 2015

(a) kinetic energy of an object; Ek = 1/2 mv²

Full specification wording

Kinetic and potential energies: (a) kinetic energy of an object; Ek = 1/2 mv²

Kinetic energy is the energy an object possesses due to its motion.

Checked against the OCR H556 specification (opens in a new tab) on 10 November 2026.

Point 1 of 3
2 key terms

Study notes

From the checked teaching notes for this specification point.

The idea

For example: A 2 kg object moving at 3 m s⁻¹ has Ek = 1/2 × 2 × 3² = 9 J. In MCQ questions, you may need to calculate kinetic energy, compare values, or identify the effect of changing mass or speed.

What earns the marks

Kinetic energy is the energy possessed by an object due to its motion.
The kinetic energy of an object is calculated using Ek = 1/2 mv², where m is mass in kg and v is speed in m s⁻¹.
Kinetic energy is directly proportional to mass and to the square of speed.
The unit of kinetic energy is the joule (J), equivalent to kg m² s⁻².
When speed doubles, kinetic energy increases by a factor of four; when mass doubles, kinetic energy doubles.

Common mistakes and how to put them right

Forgetting to square the speed.
Put it right: Square the speed before multiplying by mass and 1/2.
Using mass in grams instead of kilograms.
Put it right: Convert mass to kg by dividing by 1000 if necessary.
Confusing kinetic energy with momentum.
Put it right: Check the equation and units; momentum has units kg m s⁻¹, energy has units J.
Incorrectly calculating 1/2 mv² by multiplying m by v first and then squaring the product.
Put it right: Square the speed first, then multiply by mass and 1/2.

Exam tips

In multiple-choice questions, check whether the options involve squaring the speed; eliminate any that do not reflect the v² dependence.
If a question asks for the effect of changing speed, remember that kinetic energy is proportional to v², so a 3× increase in speed gives a 9× increase in kinetic energy.
Always write down the equation and substitute values with units to avoid errors; this also helps you check the unit of the answer.

Try the lesson

The start of the Kinetic and potential energies: (a) kinetic energy of an object; Ek = 1/2 mv² lesson, made from checked teaching notes for this specification point.

Listen

Kinetic energy is the energy an object possesses due to its motion. For an object of mass m moving at speed v, the kinetic energy Ek is given by Ek = 1/2 mv². The equation shows that kinetic energy depends on the mass and on the square of the speed. Doubling the speed quadruples the kinetic energy, while doubling the mass only doubles it. The unit of energy is the joule (J), where 1 J = 1 kg m² s⁻². When using the equation, ensure mass is in kilograms and speed in metres per second. For example, a 2 kg object moving at 3 m s⁻¹ has Ek = 1/2 × 2 × 3² = 9 J. In MCQ questions, you may need to calculate kinetic energy, compare values, or identify the effect of changing mass or speed. A strong answer earns marks for these points. Kinetic energy is the energy possessed by an object due to its motion. The kinetic energy of an object is calculated using Ek = 1/2 mv², where m is mass in kg and v is speed in m s⁻¹. Kinetic energy is directly proportional to mass and to the square of speed. The unit of kinetic energy is the joule (J), equivalent to kg m² s⁻². A common slip: Forgetting to square the speed: using v instead of v² in the equation. Instead, square the speed before multiplying by mass and 1/2. Exam tip: In multiple-choice questions, check whether the options involve squaring the speed; eliminate any that do not reflect the v² dependence.

See it

A top-down flowchart. The top node reads 'Kinetic energy: energy due to motion' and points down to a node reading 'Ek = 1/2 mv²'. From this equation node, several branches lead to: 'm is mass in kg'; 'v is speed in m s⁻¹'; 'Ek in joules (J)'; 'Square the speed first', which leads on to 'Then multiply by mass and 1/2'; 'Doubling speed quadruples Ek'; and 'Doubling mass doubles Ek'. — A flowchart showing the kinetic energy equation Ek = 1/2 mv², its quantities and units, the order of calculation, and how changing mass or speed affects kinetic energy.

Key words

Say what each word means before you turn the card.

Kinetic energy

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Joule (J)

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See every definition

Kinetic energy: The energy an object possesses due to its motion, calculated using Ek = 1/2 mv² where m is mass in kg and v is speed in m s⁻¹.
Joule (J): The unit of energy, equivalent to kg m² s⁻².

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The top node reads 'Kinetic energy: energy due to motion' and points down to a node reading 'Ek = 1/2 mv²'. From this equation node, several branches lead to: 'm is mass in kg'; 'v is speed in m s⁻¹'; 'Ek in joules (J)'; 'Square the speed first', which leads on to 'Then multiply by mass and 1/2'; 'Doubling speed quadruples Ek'; and 'Doubling mass doubles Ek'.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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