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Unit Three
Chapter 7 – Momentum
Chapter 8 – Energy
Momentum:
*inertia in motion
*mass in motion
The momentum of an object is
equal to the mass of the object
times the velocity of the object
Momentum = mass * velocity
p=m*v
The equation illustrates that
momentum is directly
proportional to an object's mass
and directly proportional to the
object's velocity.
The standard metric unit of
momentum is the kg*m/s.
Momentum is a vector quantity.
A large truck has more
momentum then a small car
going the same speed.
Why?
But a small car going very fast
can have a greater momentum
then a large truck going very
slow.
If the momentum of an object
changes, either the mass or
velocity or sometimes both
changed. Mass usually stays
the same so the velocity is
usually what changes.
And if velocity changes
acceleration occurs.
What causes acceleration?
FORCE
The greater the force acting on
the object the greater the
change in velocity and therefore
the greater momentum change
How long the force acts upon an
object is important.
Brief time, brief force small
change
Same force over an extended
period of time  greater change
So………
Both force and time are
important in changing
momentum
The quantity:
Force x time interval
is call impulse
Impulse = F x t
(Units – N · s)
Impulse = change in momentum
so…….
F x t = Δ(m x v)
Increasing Momentum
When hitting a baseball or golf
ball, you want to hit with a large
force for a long time. So you
want an impact force to be
great. That’s why your coaches
always yell at you to
“follow through.”
Decreasing Momentum
To stop a car you can hit a concrete
wall or a haystack. You will be
decreased by the same impulse
whichever you choose. Which one
do you want?
Why?
(Overhead)
The same impulse does not
mean the same amount of force
or the same amount of time. It
means the same product of
force and time.
By hitting the haystack the impact
time (the time during which your
momentum is brought to zero) is
extended. A longer impact time
decreases the force of the
impact and decreases the
deceleration.
More examples:
When you catch a baseball with
your bare hand, do you just
catch it or do you give a little
when you catch it?
When you jump off of something
do you land stiff legged or do
you bend your knees?
Air bags in cars….
extend the time required to stop
the momentum of the driver and
passenger and therefore
decrease the force
a. Boxer moves away from the
punch  increases time of impact
 reduces force of impact
b. When boxer moves toward the
punch time of impact is reduced
 Force of impact is increased
Bouncing
Impulses are greater when
objects bounce
The impulse required to bring
an object to a stop and then to
“throw it back again”
is greater than the impulse
required to merely bring the
object to a stop.
If a flower pot falls on your head
you may be in trouble. If it
bounces you might be in serious
trouble. Why?
Conservation of Momentum
Newton’s Second Law says
To accelerate an object a net
force must be applied.
For Momentum:
To change the momentum of an
object, exert an impulse on it
Either the force or impulse has
to come from outside the object
Pushing on the dashboard of
your car does not change the
momentum of the car.
(internal force)
A push on the outside of your
car could change the
momentum of the car.
(external force)
If no external force or impulse is
applied then there can be no
change in momentum.
Again………
Momentum is a vector
meaning it has magnitude and
direction
So, because momentum is a
vector, it can be canceled out or
brought to equilibrium.
Canon – Cannonball system
The canon fires the cannon ball.
The force on the cannonball is
equal to the force pushing back
rd
on the canon (3 law) so the net
force is zero. If no net force or
net impulse acts on a system,
the momentum of that system
does not change.
Conserved:
when momentum, or any other
quantity, does not change.
Law of Conservation of
Momentum
In the absence of an external
force, the momentum of a
system remains unchanged.
Collisions
Net Momentumbefore equals
Net Momentumafter
Elastic Collisions:
When objects collide without
being permanently deformed
and without generating heat.
Inelastic Collisions:
When colliding objects become
tangled or crumpled and
generate heat
You decide….
Elastic or Inelastic?
Net (mass)(Velocity)Before =
Net (mass)(velocity)After
Friction at the point of impact is
negligible so it is not taken into
account
Friction produces an impulse to
decrease momentum after a
collision……..