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Objectives
• Define force as part of an interaction. (6.1)
• State Newton’s third law of motion. (6.2)
• Given an action force, identify the reaction force.
(6.3)
• Explain why the acceleration caused by an action
force and by a reaction force do not have to be
equal. (6.4)
• Explain why an action force is not cancelled by
the reaction force. (6.5)
• Describe the horse-cart problem. (6.6)
• Explain why you can’t touch without being
touched. (6.7)
7.1 Forces and Interactions
• A force is always a part of a mutual action
that involves another force.
– Forces always occur in pairs.
7.2 Newton’s Third Law
• Describes the relationship
between two forces.
• Newton’s third law –
whenever one object
exerts a force on a second
object, the second object
exerts an equal and
opposite force on the
other object.
– Once force is called the
action force, the other,
reaction force.
Questions
1. What happens when a force exerts a force on
another object?
2. Does a stick of dynamite contain force?
3. A car accelerates along the road. Strictly
speaking, what is the force that moves the
car?
7.3 Identifying Action and Reaction
• A simple way of determining action-reaction
forces:
– First Identify the interacting objects.
• Action: Object A exerts force on Object B
• Reaction: Object B exerts force on Object A
Question
• We know the Earth pulls on the moon. Does
the moon also pull on the Earth? If so, which
pull is stronger?
7.4 Action and Reaction on Different
Masses
• The Earth and a falling boulder.
We know the boulder falls to the
Earth. But does the Earth also fall
to the boulder?
– You must draw on Newton’s second
law to help answer this question.
– Given a force exerted on a small
mass produces a greater
acceleration than the same force
exerted on a larger mass.
• Lift
– Helicopter
• Rotating blades push down on air (action) air pushed
on blades (reaction).
– Planes
• Wings receive a greater force from air underneath the
wing (action), wing provides a force back on the air
(reaction).
7.5 Defining a System
• Action and reaction
forces do not cancel
each other when
either of the forces is
external to the
system being
considered.
7.6 The Horse-Cart Problem
• If the horse in the horsecart system pushes the
ground with a greater force
than it pulls on the cart,
then there is an net force
on the horse, and the
horse-cart system
accelerates.
– It’s the force outside the
horse-cart system that
pushes the system.
7.7 Action Equals Reaction
• For every interaction
between things, there
is always a pair of
oppositely directed
forces that are equal in
strength.