Short on time? This page is the recap. For explanations, diagrams and worked examples, use the full study guide. This is the algebra-based course: no calculus is needed.
Recap
- Every force comes from an interaction between two objects, and every interaction gives two forces: one on each object.
- Newton’s third law: F_(A on B) = −F_(B on A). The pair is equal in size, opposite in direction, the same type, simultaneous, and acts on different objects.
- The pair has equal size whatever the objects’ masses, speeds or materials. The effects can be very different.
- Forces between parts of a chosen system are internal. They cancel in pairs, so they cannot change the motion of the system’s centre of mass.
- Tension is the net pull that neighbouring pieces of a string exert on each other.
- In Physics 1, the only force between objects that do not touch is gravity.
Key relationships
| Relationship | Meaning | Use it to |
|---|---|---|
| F_(A on B) = −F_(B on A) | equal size, opposite direction | name and size a partner force |
| “[type] force of A on B” ↔ “[type] force of B on A” | swap the objects, keep the type | find the pair in words |
| sum of internal forces = 0 | pairs inside the system cancel | explain why the centre of mass is unaffected |
| ideal string: T the same at all points | negligible mass, no stretch | use one tension value for the whole string |
| ideal pulley | negligible mass, frictionless axle | change a string’s direction without changing T |
Assumptions behind the numbers
- Strings, ropes and cables are ideal (massless, no stretch) unless a question says they have mass.
- Pulleys are ideal unless stated.
- A rope or chain with mass can carry different tensions at different points; describe this in words only.
- Near Earth’s surface, use g = 9.8 m/s² for gravitational forces.
Mistakes to avoid
- Pairing weight with the normal force. Both act on the same object; they are not a pair.
- Cancelling a pair. Only forces on the same free-body diagram can cancel.
- “The heavier object pushes harder.” The forces are equal; the accelerations differ.
- Doubling tension when both ends are pulled. Two 80 N pulls give 80 N of tension.
- Putting both forces of a pair on one free-body diagram. Each diagram shows only forces on that object.
- Calling the second force a “reaction” that comes later. Both forces exist at the same time.
- Using internal forces to move a system. Only external forces change the centre of mass’s motion.
Quick self-check
- A hammer hits a nail. What is the third-law partner of the force of the hammer on the nail? (The force of the nail on the hammer: same size, opposite direction, same type.)
- A 0.80 kg cart and a 2.4 kg cart push apart with a light spring between them. How do the forces on them compare, and which cart’s velocity changes more? (Equal forces; the 0.80 kg cart’s velocity changes 3 times as much.)
- A heavy rope hangs from a hook. Where is the tension greatest? (At the top: that part holds up all the rope below it.)
Next: practice questions.