Theme A — Space, Time and Motion
A.2.4 Newton’s Third Law — IB Physics HL Revision Notes & Topical Questions
5 min read 9 parts Lesson 15 of 63 1 quiz
Curriculum Equation Key قوانين نيوتن للحركة - Newton Laws of Motion FormulasFast Exam Solution
F = m × a | W = m × g | F₁₂ = -F₂₁
Word Equation & Conditions: القوة المحصلة = الكتلة × التسارع | Force = Mass × Acceleration
Location & Specifications: الميكانيكا الكلاسيكية والتحصيلي وفيزياء 3 ثانوي و IGCSE Physics.
Lesson summary
A.2.4 Newton’s Third Law — IB PHYSICS SL notes: State Newton's third law, identify interaction pairs and distinguish them from balanced forces on a single body.Newton's third law — if object A exerts a force on object B, then B exerts a force on A that is equal in magnitude and o…
- Conditions for a pair
- Simple labelling
- Book on a table
- Free-body diagram clue
- No cancelling
- Car and lorry collision
Section 1 of 9The law
Newton's third law
- Newton's third law if object A exerts a force on object B, then B exerts a force on A that is equal in magnitude and opposite in direction.
Interaction pairs
- Interaction pairs forces arise in pairs, also called third-law pairs or action–reaction pairs.
Conditions for a pair
- Conditions for a pair all four must hold.
- The forces are the same type.
- They have the same magnitude.
- They act in opposite directions.
- They act on different objects.
Timing
- Timing both forces exist at the same time and neither causes the other.
Section 2 of 9Identifying a partner force
Identifying a partner force
Method
- Method swap the two bodies and keep the type of force.
Gravity
- Gravity the Earth pulls a book down and the book pulls the Earth up, both gravitational forces.
Contact
- Contact a table pushes up on a book and the book pushes down on the table, both normal contact forces.
Friction
- Friction a foot pushes back on the ground and the ground pushes the foot forwards.
Simple labelling
- "A pushes or pulls on B, and B pushes or pulls on A".
Section 3 of 9Pairs compared with balanced forces
| Feature | Third-law pair | Balanced forces on one body |
|---|---|---|
| Number of bodies | Two different bodies | One body |
| Type of force | Same type | Can differ |
| Equal and opposite | Always | Only in equilibrium |
| Cancel each other | No | Yes |
Book on a table
- Book on a table weight and normal force are equal and opposite but act on the same body and differ in type, so they are not a pair.
- They are equal because of the first law, as the book is in equilibrium.
Free-body diagram clue
- Free-body diagram clue a diagram of forces on one body shows balanced forces, not a third-law pair.
Section 4 of 9Why things accelerate
No cancelling
- No cancelling the pair forces act on different bodies, so each body responds to the resultant on it.
Car and lorry collision
- Car and lorry collision the forces are equal and opposite, but the car has much less mass so its acceleration F/m is larger.
Rocket
- Rocket it pushes exhaust gas backwards and the gas pushes the rocket forwards.
Swimmer
- Swimmer pushes water backwards and the water pushes the swimmer forwards.
Walking
- Walking you push backwards on the ground and the ground pushes you forwards;
- the Earth's acceleration is unmeasurable.
Section 5 of 9A person standing on a floor
Gravitational pair
- Earth pulls the person down and the person pulls the Earth up.
Normal pair
- Normal pair the floor pushes the person up and the person pushes the floor down.
Forces on the person
- Forces on the person weight and normal force add to zero because the person is not accelerating.
Jumping
- Jumping the push on the floor increases, so the floor pushes back harder and the person accelerates upwards.
Earth's response
- Earth's response with a mass of 6 × 1024 kg its acceleration is too small to notice.
Section 6 of 9Links and applications
Conservation of momentum
- Conservation of momentum in a collision the forces are equal and opposite for the same time,
- so the impulses are equal and opposite and the total momentum is unchanged.
Tension in a rope
- Tension in a rope it is equal at both ends of a light rope.
Contact forces
- Contact forces the forces between two surfaces are equal in size.
Problem solving
- Problem solving use the law to relate the force on one body to the force on another, such as two blocks pushed in a line.
Section 7 of 9More from the specification
1
Newton's third law
- Newton's third law when A exerts a force on B, B exerts an equal and opposite force on A, of the same type and acting on different objects.
2
Force pairs
- Force pairs they act on different bodies, so they never cancel each other.
3
Motion consequences
- Motion consequences a rocket gains velocity because the exhaust gas is pushed backward.
4
Circular motion
- Circular motion centripetal force toward the centre keeps a body moving in a circle at constant speed with changing velocity.
5
Fluid example
- Fluid example a fan or jet pushes fluid one way and is pushed in the opposite direction.
6
Fluid resistance
- Drag force from a fluid opposing motion.
7
Buoyancy
- Another name for upthrust.
8
Centripetal force
- Net force toward the centre, mv2/r.
9
Static friction
- Friction between surfaces at rest relative to each other; fs ≤ μsFN.
10
Circular motion
- Motion along a circular path with a force towards the centre.
11
Kinetic energy
- K = ½mv2, energy of motion.
12
Angular speed
- Magnitude of the angular velocity.
13
Centripetal acceleration
- Ac = v2/r toward the centre.
14
Linear speed
- The distance travelled along the circle per unit time, v = ωr.What the symbols mean: v = speed (m s−1) · ω = angular speed (rad s−1) · r = radius (m) · T = period (s)
15
Velocity
- Rate of change of displacement.
16
Constant
- Not changing.
17
Linear
- A graph that is a straight line through the origin, so resistance is constant.
18
Radius
- Distance from the centre to the circular path.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Newton's third law | If A exerts a force on B, B exerts an equal and opposite force of the same type on A |
| Interaction pair | The two forces of equal size and opposite direction acting on two different bodies |
| Normal contact force | Contact force perpendicular to the surfaces in contact |
| Weight | Gravitational pull of the Earth on a body |
| Equilibrium | State in which the resultant force on a body is zero |
Section 9 of 9Exam technique
1
Saying the third-law forces cancel — they act on different bodies.
2
Treating weight and the normal force on a resting object as a pair — they act on the same body.
3
Believing the larger object exerts a larger force in a collision — the forces are equal.
4
Pairing forces of different types, such as contact with gravitational — a pair is the same type.
5
Thinking the reaction occurs after the action — both exist simultaneously.
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