Download Action/Reaction

Cornell Notes 3.3
Newton’s Laws
November 29, 2011
Pages 91 & 93
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.
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.
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?
Motion in Space!
• If you had an object moving in space, and
there was no unbalanced force acting on
it, it would keep the same speed and
velocity forever!
Ten hours
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.”
Force, mass and acceleration
• Force causes acceleration, and mass
resists acceleration.
• Newton’s Second Law relates the force
on an object, the mass of the object, and
its acceleration.
• Force causes acceleration, and mass
resists acceleration.
Force and Mass
• In other words, the bigger something is,
the harder it is to accelerate
Easy
HARD!
F=m*a
(Force = Mass times
Acceleration)
Forces always come in matched
pairs
•
•
Newton’s Third Law
(action-reaction)
applies when a force is
placed on any object,
such as a basketball.
It says that forces
always come in
matched pairs
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.
The Third Law: Action/Reaction
•
•
Here, 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.
Action and reaction
•
When sorting out
action and reaction
forces it is helpful to
examine or draw
diagrams.
Here, one force acts on the ________________,
and the other force acts on the _______________.
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.
Collisions
•
•
Newton’s third law tells us that any time two objects hit
each other, they exert equal and opposite forces on each
other.
However, the effect of the force is not always the same.
Collisions
• When a large truck
hits a small car, the
forces are equal.
• However, the small
car experiences a
much greater
change in velocity
much more rapidly
than the big truck. Which vehicle ends up
with more damage?
Momentum
• The energy of crashing objects is known
as their momentum.
• You can figure out an object’s momentum
by multiplying its mass times its velocity
(Momentum=m*v)
• In all collisions, momentum is conserved.
(So the total momentum remains the
same)