Newton's laws Cambridge International AS & A Level Physics revision
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In plain words
Three laws explain why things move the way they do. Nothing changes its motion unless a force makes it. The bigger the resultant force, the bigger the acceleration. And forces always come in pairs: push on a wall and the wall pushes back on you just as hard.
4 things to know
- First law: a body stays at rest or moves at constant velocity unless a resultant force acts on it. Second law: resultant force = rate of change of momentum, which for constant mass is F = ma.
- Third law: if A exerts a force on B, then B exerts an equal and opposite force on A. The two forces are the same type, and act on different bodies.
- Draw a free-body diagram showing only the forces acting on the one object, then apply F = ma along the direction of motion.
- Mass is the property of an object that resists changes to its motion. Weight is the gravitational force on it: W = mg, where g is the gravitational field strength (9.81 N/kg near the Earth's surface).
Worked example
A 70 kg person stands in a lift accelerating upwards at 2.0 m/s². Find the force from the floor on the person.
- Forces on the person: R upwards and weight mg downwards.
- Resultant upwards = ma: R − mg = ma, so R = m(g + a).
- R = 70 × (9.81 + 2.0) = 827 N.
Tips and tricks
- F in F = ma is the resultant force. Find that first.
- A third-law pair never acts on the same object. Weight and the normal force on a book are not a pair.
It lands in your notebook with its questions as flashcards.
Newton's laws: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
A body moves at constant velocity. What must be true?
Forces may act, but they balance.
What is the resultant force on a 4.0 kg mass accelerating at 3.0 m/s²?
F = ma.
Which is true of a Newton's third law pair of forces?
Each acts on one of the two bodies involved.
A person stands in a lift moving up at constant speed. How does the force from the floor compare with their weight?
Constant velocity means no resultant force.
In Newton's second law, force is equal to the rate of change of what?
F = Δp ÷ Δt.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 6 marks. Write your answers on paper, then check them.
Newton's laws
Cambridge International AS & A Level Physics 9702 · 6 marks · papermunch.org
Name ______________________________ Date ______________
A 1200 kg car has a driving force of 3000 N and total resistive forces of 600 N. Find its acceleration.[2]
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2.0 m/s². 2400 ÷ 1200.
A block slides down a frictionless slope at 30° to the horizontal. Find its acceleration.[2]
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4.9 m/s². g sin 30°.
A book rests on a table. State the third-law partner of the book's weight.[2]
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The gravitational pull of the book on the Earth (upwards on the Earth).
Answers: Newton's laws
- 1. 2.0 m/s². 2400 ÷ 1200.
- 2. 4.9 m/s². g sin 30°.
- 3. The gravitational pull of the book on the Earth (upwards on the Earth).