Newton's Third Law
Newton's Third Law
- Forces never come alone. Every force is one half of a pair between two objects.
- This law is short to state and easy to misuse, so it appears in many papers.
The law and the four checks
- Newton's third law: when two bodies interact, the forces they exert on each other are equal in magnitude and opposite in direction.
- If object A pushes object B, then B pushes A back with the same size of force the other way. The two forces appear together, at the same time. Neither one causes the other.
Definition
2 marksWhat is meant by Newton's third law?
Model answer: When two bodies interact, the forces they exert on each other are equal in magnitude and opposite in direction.
A true third-law pair passes all four checks:
- the two forces act on different objects (one on each);
- they are equal in magnitude;
- they are opposite in direction;
- they are the same type of force (both gravitational, both contact, both magnetic, and so on).
- A person standing on the ground is part of two separate pairs. Gravity pair: the Earth pulls the person down, and the person pulls the Earth up. Contact pair: the ground pushes the person up, and the person pushes the ground down.
- Each pair acts across two objects: person and Earth for gravity; person and ground for contact. That is exactly what check 1 asks for.
The two forces of a third-law pair act on different objects, so they do not cancel in the force balance for either object. When both objects are treated as one system, the pair is internal and its vector sum is zero.
The most common wrong answer
- A book rests on a table. Its weight (down) and the table's contact force on it (up) are equal and opposite. Students call them a third-law pair. They are not.
- Both forces act on the same object (the book), which fails check 1. They are also different types (one gravitational, one contact), which fails check 4.
- The real partner of the book's weight is the book's gravitational pull on the Earth. The real partner of the table's push is the book's push on the table.
Common mistake
“Weight and the normal contact force are a third-law pair.” They act on the same object and are different types of force. Equal and opposite is not enough: run all four checks.
How the exam asks it
- Rockets: the engine pushes exhaust gas backwards, and the gas pushes the rocket forwards. The pair is “force of rocket on gas” and “force of gas on rocket”.
- Car wheels: the tyre pushes backwards on the road, and the road pushes forwards on the tyre. That forward friction force is what drives the car.
- Collisions: when X hits Y, the force on Y from X is equal in magnitude and opposite in direction to the force on X from Y, and the two forces act for the same time.
Your turnAdapted from 9702/22/M/J/21 Q2(b)
A ball collides with the ground. State how Newton's third law applies to this collision. (9702/22/M/J/21 Q2(b))
Show worked answer
The force from the ground on the ball is equal in magnitude and opposite in direction to the force from the ball on the ground.