Newton’s 3rd Law

Newton’s 3rd LawNewton’s 2nd Law is:

A quantitative description of how forces affect motion.• BUT: Where do forces come from?

– Answer this with EXPERIMENTS!! – The results show that forces applied to an object are

ALWAYS applied by another object

Newton’s 3rd Law:“Whenever one object exerts a force F on a second object, the second object exerts an equal and opposite force -F on the first object.”– Law of Action-Reaction: “Every action has an equal &

opposite reaction”. (Note that action-reaction forces act on DIFFERENT objects!)

Another Statement of Newton’s 3rd Law

“If two objects interact, the force F12 exerted by object 1 on object 2 is equal in magnitude & opposite in direction to the force F21 exerted by object 2 on object 1.” As in the figure

Example: Newton’s 3rd LawWhen a force is exerted on

an object, that force is caused by another object.

Newton’s 3rd Law:“Whenever one object exerts a force on a second object, the second exerts an equal

force in the opposite direction on the first.”

If your hand pushes against the edge of a desk (red force vector), the desk pushes back against your hand (purple

force vector; the two colors tell us that this force acts on a

DIFFERENT object).

Newton’s 3rd LawAlternative Statements

1. Forces ALWAYS occur in pairs

2. A single isolated force CANNOT exist3. The “action force” is equal in magnitude

to the “reaction force” & opposite in direction.a. One of the forces is the “action force”, the

other is the “reaction force”b. It doesn’t matter which is considered the

“action” & which the “reaction”c. The action & reaction forces MUST ACT ON

DIFFERENT OBJECTS & be of the same type.

Action-Reaction Pairs: Act on Different Objects

The key to correct application of

Newton’s 3rd Law is:

THE FORCES ARE EXERTED ON DIFFERENT

OBJECTS. Make sure you don’t use them as if they were acting on the same object.

ExampleAn ice skater pushes against a railing. The

railing pushes back & this force causes her to move away.