Download 5.3 Newton`s Third Law

Chapter Five: Newton’s
Laws of Motion
• 5.1 Newton’s First Law
• 5.2 Newton’s Second Law
• 5.3 Newton’s Third Law
Investigation 5A
Force and Acceleration
• What happens
when force is
applied to
something that
can move?
5.1 Force changes motion
•
A force is a push or pull, or any action that
is able to change motion.
5.1 Law of inertia
•
•
•
Newton’s first law says that objects
continue the motion they already have
unless they are acted on by a net force.
If the net force is zero, an object at rest
will stay at rest.
If an object is acted upon by unbalanced
forces, its motion will change.
5.1 Force changes motion
•
Forces can be used to increase or
decrease the speed of an object, or to
change the direction an object is moving.
5.1 Law of inertia
•
•
Inertia is the
property of an
object that resists
changes in motion.
Objects with more
mass have more
inertia and are
more resistant to
changes in their
motion.
Which ball has
more inertia?
5.1 The net force
•
•
Newton’s first law is
often written in terms
of the net force:
“An object at rest will
stay at rest and an
object in motion will
continue in motion at
constant velocity
UNLESS there is a
net force.”
5.2 Newton’s Second Law
• The second law is the connection between
force, mass, and motion.
5.2 Three main ideas of the
second law
1. Acceleration is the result of unbalanced
forces.
2. A larger force makes a proportionally
larger acceleration.
3. Acceleration is inversely proportional to
mass.
5.2 Acceleration and force
• The second law says
that acceleration is
proportional to
force.
• If force is increased
or decreased,
acceleration will be
increased or
decreased by the
same factor.
5.2 Acceleration and force
• Acceleration is always in the same direction
as the net force.
5.2 Acceleration and mass
• Acceleration is
inversely
proportional to
mass.
• When the forces
stay the same,
increasing mass
decreases the
acceleration.
• The stronger the
force on an
object, the
greater its
acceleration.
– Force is directly
proportional to
acceleration.
– If twice the force
is applied, the
acceleration is
twice as great.
• The greater the
mass, the
smaller the
acceleration for a
given force.
– Mass is inversely
related to force.
– An object with
twice the mass
will have half the
acceleration if
the same force is
applied.
5.3 Newton’s Third Law
•
Newton’s Third Law
(action-reaction)
applies when a force
is placed on any
object, such as a
basketball.
5.3 The Third Law: Action/Reaction
•
Newton’s Third Law
states that every action
force creates a reaction
force that is equal in
strength and opposite in
direction.
•
There can never be a
single force, alone, without
its action-reaction partner.
5.3 The Third Law: Action/Reaction
•
•
One force acts on the
ball, and the other force
acts on the hand.
It doesn’t matter which
force you call the action
and which the reaction.
The forces do not
cancel because we can
only cancel forces
acting on the same
object.
5.3 Action and reaction
•
When sorting out
action and reaction
forces it is helpful to
examine or draw
diagrams.
One force acts on the ________________, and
the other force acts on the _______________.
5.3 Action and reaction forces
Below are some guidelines to help you
sort out action and reaction forces:
1. Both are always present whenever any force
appears.
2. They always have the exact same strength.
3. They always act in opposite directions.
4. They always act on different objects.
5. Both are real forces and can cause changes
in motion.
5.3 Collisions
•
•
Newton’s third law tells us that any time two
objects hit each other, they exert equal and
opposite forces on each other.
The effect of the force is not always the same.
5.3 Collisions
• When a large truck
hits a small car, the
forces are equal.
• The small car
experiences a
much greater
change in velocity
much more rapidly
than the big truck.
Which vehicle ends up
with more damage?
Physics Connection
Rockets: Out of This
World Travel
• Robert Goddard (1882 to
1945),an American
scientist, concluded that it
was possible to travel to
space. He launched the
first liquid-fueled rocket in
1926.