Newton's Laws of Motion — A-Level Physics Revision

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What is Newton's Laws of Motion?

This topic introduces the three fundamental laws of motion formulated by Isaac Newton, which form the basis of classical mechanics. The first law deals with inertia, the second law relates net force, mass, and acceleration (F=ma), and the third law describes action-reaction force pairs. These laws are essential for analysing why and how objects move, from everyday situations to the motion of planets.

Board notes: Newton's Laws of Motion are a fundamental part of all A-Level Physics courses (AQA, Edexcel, OCR). The application of F=ma to various scenarios, including lifts and connected particles, is a common theme. All boards expect a clear understanding of the definition of the second law in terms of momentum.

Worked example

Common mistakes

• 1Incorrectly identifying Newton's third law pairs. A classic mistake is pairing the weight of an object with the normal contact force; these forces act on the same object, whereas action-reaction pairs always act on two different interacting objects.
• 2Assuming F=ma is the complete definition of Newton's second law. The full law states that force is the rate of change of momentum (F = Δp/Δt), which is crucial for problems involving variable mass, like a rocket expelling fuel.
• 3Believing that a net force is required to maintain motion. Newton's first law clarifies that a net force is only needed to cause acceleration (a change in velocity), not to sustain a constant velocity.

Newton's Laws of Motion exam questions

Exam-style questions for Newton's Laws of Motion with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.

Frequently asked questions

• If action and reaction forces are equal and opposite, why don't they cancel out?

  They don't cancel out because they act on different objects. For an object to accelerate, we only consider the forces acting *on that object*. The reaction force acts on the other object in the interaction.

• What is the difference between mass and weight?

  Mass is the amount of matter in an object and a measure of its inertia, measured in kilograms (kg). Weight is the force of gravity acting on an object (Weight = mass × gravitational field strength), measured in Newtons (N).