3rd law

NEWTON'S THIRD LAW

•FORCES ALWAYS OCCUR IN PAIRS.

•You can’t touch an object without
being touched by it!

If every action force has an
equal/opposite reaction force, then how
is it that things move?

● Why isn’t every situation Balanced

Forces?

•Action/reaction forces act on
different objects

•THEY DO NOT CANCEL EACH
OTHER OUT

Balanced Forces

•Example:

Net Force = 0 N

They cancel each other out b/c
they are acting on the same box!

5 Newtons

5 Newtons

What is the action force?

•Person pushes on the wall.

What is the reaction
force?
•Wall pushes back on the
person.

Person! Because they have

a smaller mass.

Example 1(pushing on wall)

What moves?

Example 2(swimmer)

What is the action force?
Hand pushes water.

What is the reaction

force?

Water pushes on

hand.

Equal forces do not have
equal effects
•Mass is the key to acceleration

•Recall F = m x a

Kicking a ball results in a force on the ball
from your foot, and an equal and opposite
force on your foot from the ball.

Ball

Foot

F = ma F = ma

The mass of the foot/body is larger so it doesn’t experience a large acceleration.

Example 3 (Balloon)

What is the action
force?
•Balloon pushes air out.

What is the reaction
force?
•Air pushes on balloon
in opposite direction

Example 4(rocket)

What is the action
force?
•Shuttle pushes gas
downward.

What is the reaction
force?
•Gas pushes shuttle
upward.

ACTION - REACTION PAIRS