What mistakes do students make with Kinetic and potential energies?

Assuming energy is lost when it is only transferred between forms. In the absence of resistive forces, total mechanical energy is constant. Forgetting that mass cancels when equating mgh and 1/2 mv², leading to v = √(2gh). Recognise that the final speed is independent of mass. Including air resistance in calculations when the question states it is negligible. Only include resistive forces if explicitly mentioned.

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

In multiple-choice questions, look for options that conserve total energy; eliminate any that suggest energy is created or destroyed. If a question involves a falling object, you can often use mgh = 1/2 mv² to find speed without knowing the mass. Check whether the system is isolated; if friction or air resistance is present, mechanical energy is not conserved.

OCR · A-Level · Physics

Course version: H556 · Version from Sept 2015

(c) the exchange between gravitational potential energy and kinetic energy.

Full specification wording

Kinetic and potential energies: (c) the exchange between gravitational potential energy and kinetic energy.

In a closed system where only gravity acts, gravitational potential energy (GPE) and kinetic energy (KE) can be exchanged.

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

Point 3 of 3

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

Gravitational potential energy can be converted into kinetic energy and vice versa.
In the absence of resistive forces, the total mechanical energy (sum of GPE and KE) is conserved.
The exchange is described by equating the loss in GPE to the gain in KE: mgh = 1/2 mv².
The speed of a falling object can be found from v = √(2gh) when starting from rest and ignoring air resistance.
Energy conservation applies to systems where only conservative forces (like gravity) do work.

Common mistakes and how to put them right

Assuming energy is lost when it is only transferred between forms.
Put it right: In the absence of resistive forces, total mechanical energy is constant.
Forgetting that mass cancels when equating mgh and 1/2 mv², leading to v = √(2gh).
Put it right: Recognise that the final speed is independent of mass.
Including air resistance in calculations when the question states it is negligible.
Put it right: Only include resistive forces if explicitly mentioned.
Mixing up the direction of energy transfer.
Put it right: Identify whether the object is gaining or losing height.

Exam tips

In multiple-choice questions, look for options that conserve total energy; eliminate any that suggest energy is created or destroyed.
If a question involves a falling object, you can often use mgh = 1/2 mv² to find speed without knowing the mass.
Check whether the system is isolated; if friction or air resistance is present, mechanical energy is not conserved.

Try the lesson

The start of the Kinetic and potential energies: (c) the exchange between gravitational potential energy and kinetic energy. lesson, made from checked teaching notes for this specification point.

Listen

In a closed system where only gravity acts, gravitational potential energy (GPE) and kinetic energy (KE) can be exchanged. As an object falls, it loses GPE and gains KE; as it rises, it gains GPE and loses KE. If air resistance is negligible, the total mechanical energy (GPE + KE) remains constant. For example, a ball dropped from height h converts all its initial GPE (mgh) into KE just before impact, so 1/2 mv² = mgh, giving v = √(2gh). In MCQ questions, you may be asked to identify energy changes, calculate speeds using conservation of energy, or compare energies at different points. A strong answer earns marks for these points. Gravitational potential energy can be converted into kinetic energy and vice versa. In the absence of resistive forces, the total mechanical energy (sum of GPE and KE) is conserved. The exchange is described by equating the loss in GPE to the gain in KE: mgh = 1/2 mv². The speed of a falling object can be found from v = √(2gh) when starting from rest and ignoring air resistance. A common slip: Assuming energy is lost when it is only transferred between forms. Instead, in the absence of resistive forces, total mechanical energy is constant. Exam tip: In multiple-choice questions, look for options that conserve total energy; eliminate any that suggest energy is created or destroyed.

See it

A flowchart with two starting branches. The first branch begins at a node labelled 'Object falls', which leads to two nodes: 'Loses gravitational potential energy' and 'Gains kinetic energy'. The second branch begins at 'Object rises', leading to 'Gains gravitational potential energy' and 'Loses kinetic energy'. All four of these nodes point to a single node reading 'Total mechanical energy (GPE + KE) constant if air resistance negligible'. That node leads to 'Loss in GPE = gain in KE: mgh = 1/2 mv²', which leads to the final node 'v = √(2gh) for object falling from rest'. — Shows how gravitational potential energy and kinetic energy are exchanged as an object falls or rises, with total mechanical energy conserved when air resistance is negligible.

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 flowchart with two starting branches. The first branch begins at a node labelled 'Object falls', which leads to two nodes: 'Loses gravitational potential energy' and 'Gains kinetic energy'. The second branch begins at 'Object rises', leading to 'Gains gravitational potential energy' and 'Loses kinetic energy'. All four of these nodes point to a single node reading 'Total mechanical energy (GPE + KE) constant if air resistance negligible'. That node leads to 'Loss in GPE = gain in KE: mgh = 1/2 mv²', which leads to the final node 'v = √(2gh) for object falling from rest'.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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