1.2.4Tools and Inquiry — IB Physics skills
Newton’s Third Law — IB Physics SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 25 of 33 Unit 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.
Section 1 of 5The Third Law
Forces always occur in pairs because they come from an interaction between two objects.
Statement
- Statement If object A exerts a force on object B, then B exerts an equal and opposite force on A.
Same type
- Same type The two forces in a pair are always the same kind, such as both gravitational or both electromagnetic contact forces.
Different objects
- Different objects One force of the pair acts on each object, so they never act on the same body.
Same time
- Same time The two forces exist at the same instant and are equal in magnitude at every moment of the interaction.
Section 2 of 5Identifying Pairs
Finding the true partner of a force is the key skill.
1
Name the force using the form: force on object A by object B, for example the force on the book by the table.
2
Swap the objects to name the partner: the force on the table by the book, equal in size and opposite in direction.
3
Check that both forces are of the same type, so that the partner of a gravitational force is also gravitational.
4
Confirm the two forces act on different objects, otherwise they are not a third-law pair.
5
Remember that a book's weight and the normal reaction from the table are not a third-law pair, since both act on the book.
Section 3 of 5Consequences
Consequences
Walking
- Walking The foot pushes backwards on the ground and the ground pushes forwards on the foot, producing the forward motion.
Rocket
- Rocket Hot gas is pushed backwards by the rocket, and the gas pushes the rocket forwards with an equal force.
Different accelerations
- Different accelerations Equal forces on bodies of different mass give different accelerations because a = F/m.
No cancellation
- No cancellation The forces do not cancel because they act on different bodies, so each body can accelerate.
| Force | Third-law partner |
|---|---|
| Earth pulls on a ball | Ball pulls on Earth |
| Hand pushes on wall | Wall pushes on hand |
| Tyre pushes on road | Road pushes on tyre |
Section 4 of 5Worked Example
Question
- A 60 kg skater pushes a 80 kg skater with a force of 120 N on smooth ice.
- Find the acceleration of each skater.
- By the third law, each skater feels a force of 120 N, in opposite directions.
- Skater of mass 60 kg: a = 120 / 60 = 2.0 m s⁻².
- Skater of mass 80 kg: a = 120 / 80 = 1.5 m s⁻².
Answer The lighter skater accelerates at 2.0 m s⁻² and the heavier skater at 1.5 m s⁻², in opposite directions.
Section 5 of 5Exam Checklist & Tips
1.2.4 – Key Points for the Exam
- Interaction pair Two forces of equal size and opposite direction acting on different objects.
- Normal reaction The perpendicular contact force exerted by a surface on an object resting on it.
- Contact force A force that acts only when two objects touch.
- Newton's third law Forces between two objects are equal in size and opposite in direction.
Common Mistakes
- Choosing the weight and normal reaction on one object as a third-law pair.
- Claiming the forces cancel so nothing can move; they act on different bodies.
- Saying the larger object exerts a larger force than the smaller object in a collision.
- Naming partner forces of different types, such as gravity paired with a contact force.
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