Download Newton`s Third Law and Momentum

Newton’s Third Law and
Momentum
Section 12.3
Review First and Second Laws
1. An object will remain at rest or in
motion at constant velocity unless
acted upon by a net force.
2. The acceleration of an object is
directly proportional to force and
inversely proportional to the mass
Contrasting Newton’s Laws
• The first and second laws of
motion apply to single objects.
• In contrast, the third law of
motion deals with pairs of
objects.
Forces and Interactions
When you push on the wall, the wall pushes
on you.
Forces and Interactions
The interaction that drives the nail is the
same as the one that halts the hammer.
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 first object.
Newton’s Third Law
Newton’s third law describes the relationship
between two forces in an interaction.
• One force is called the action force.
• The other force is called the reaction force.
• Neither force exists without the other.
• They are equal in strength and opposite in
direction.
• They occur at the same time
(simultaneously).
When this bumper car
collides with another car,
two forces are exerted.
Each car in the collision
exerts a force on the other.
Newton’s Third Law
Action and Reaction Forces
• The force your bumper car exerts on the
other car is the action force.
• The force the other car exerts on your car
is the reaction force.
• These two forces are equal in size and
opposite in direction.
Newton’s Third Law
When the girl jumps to shore, the boat moves backward.
Identifying Action and Reaction
When action is A exerts force on B, the reaction is simply B exerts
force on A.
Identifying Action and Reaction
When action is A exerts force on B, the reaction is simply B exerts
force on A.
Action and Reaction on Different Masses
A given force exerted on a
small mass produces a greater
acceleration than the same
force exerted on a large mass.
Action and Reaction on Different Masses
Earth is pulled up by
the boulder with just as
much force as the
boulder is pulled down
by Earth.
Because Earth has a
huge mass, we don’t
sense its infinitesimally
small acceleration.
Action and Reaction on Different Masses
The balloon recoils
from the escaping air
and climbs upward.
A rocket accelerates in
much the same way—
it continually recoils
from the exhaust
gases ejected from its
engine.
Action and Reaction on Different Masses
Using Newton’s third law, we can understand
how a helicopter gets its lifting force.
• The whirling blades force air particles
downward (action).
• The air forces the blades upward (reaction).
• This upward reaction force is called lift.
• When lift equals the weight of the craft, the
helicopter hovers in midair. When lift is
greater, the helicopter climbs upward.
Momentum
• Momentum (p) measures a moving
object’s resistance to changes in its
motion.
• Includes effects of speed and
direction as well as mass.
Momentum
•A moving truck is harder to stop than a
car moving at the same speed because
the truck has more mass.
The truck has more momentum than the
car. By momentum, we mean inertia in
motion.
•A fast car can have more momentum
than a slow truck.
•A truck at rest has no momentum at all.
Momentum
• Stopping a
fast-moving
object is
harder than
stopping a
slow-moving
one.
Changing Momentum
When you push with the same force for twice
the time, you impart twice the impulse and
produce twice the change in momentum.
Changing Momentum
Increasing Momentum
To increase the momentum of an object,
apply the greatest force possible for as long
as possible.
A golfer teeing off and a baseball player
trying for a home run do both of these things
when they swing as hard as possible and
follow through with their swing.
Changing Momentum
Decreasing Momentum
If you were in a car that was out of control
and had to choose between hitting a haystack
or a concrete wall, you would choose the
haystack.
Physics helps you to understand why hitting a
soft object is entirely different from hitting a
hard one.
Changing Momentum
If the change in momentum occurs over a long
time, the force of impact is small.
Changing Momentum
If the change in momentum occurs over a short
time, the force of impact is large.
Conservation of Momentum
Objects within a closed system can exert
forces on one another.
According to the law of conservation of
momentum, if no net force acts on a
system, then the total momentum of the
system does not change.
Conservation of Momentum
In each collision, the total momentum of
the train cars does not change—
momentum is conserved.
Conservation of Momentum
In each collision, the total momentum of
the train cars does not change—
momentum is conserved.
Conservation of Momentum
In each collision, the total momentum of
the train cars does not change—
momentum is conserved.