3rd law

NEWTON'S THIRD
LAW OF MOTION

For every action force,

there is an equal (in size)
and opposite (in direction)

reaction force

NEWTON'S THIRD LAW

•FORCES ALWAYS OCCUR IN PAIRS.

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

Multiple Choice

Forces always occur ....

• in a downward direction

• in an upwards direction

• by themselves

• in pairs

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

Multiple Choice

Why don't action-reaction forces cancel out and have a net force of zero?

• The are not equal in size.

• They go in the same direction so are added together

• They act on differect objects

• They are an exception to the laws of physics.

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.

Multiple Choice

Which object experiencing force pairs will experience the most acceleration (or movement)?

• The most massive

• The least massive

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

Match

Match the following action reaction pairs

• hammer head onto nail

• person on box

• feet onto trampoline

• feet onto boat

nail onto hammer

box on person

trampoline onto feet

boat onto feet

• nail onto hammer

• box on person

• trampoline onto feet

• boat onto feet