Transcript Momentum is a measure of inertia in motion. Momentum is

Momentum is a
measure of inertia in
motion. Momentum is
equal to mass
multiplied by velocity.
p = mv
A 2250 kg pickup
has a velocity of
25 m/s east. What
is the momentum
of the truck?
A moving object
can have a large
momentum if it has a
large mass, a high
speed, or both.
Can you think of a
case where a roller
skate and a truck
would have the
same momentum?
If momentum
changes, either
the mass of the
object or its velocity
or both change.
In most cases the
velocity changes,
not the mass.
A change in velocity is
acceleration, which is
caused by only one
thing:
an unbalanced force.
The greater the force,
the greater the
acceleration and the
greater the change in
momentum.
The length of time the
force is applied is also
important. The longer
the force is applied the
greater the change in
momentum.
Force times
time interval is
called impulse.
Impulse = Ft
Remember F = ma,
and a = v/t.
Therefore:
F = m v/t,
or: Ft = mv
Ft = mv,
or Ft = mv
Impulse is equal
to change in
momentum.
A 1400 kg car moving west
with a velocity of 15 m/s
collides with a utility pole and
is brought to rest in 0.30 s.
Find the magnitude of the
force exerted on the car
during the collision.
To increase the
momentum of an
object, one should
apply a large force
for a long period of
time.
Decreasing
momentum also
requires an
impulse.
mv = Ft
A great mv can be
accomplished with
little force (F) if the
force is applied over
a large time (t).
If you lost control of
your car, would you
rather run into a
brick wall or a
stack of hay?
Other examples of
extended “t”: a padded
dashboard, airbags,
bent-knee landings,
mats for gymnasts,
a glass dish dropped
on a carpeted floor.
When the time
of impact is
extended,
it reduces the
force of impact.
Why is hail
more damaging
than rain?
Bouncing objects
require a greater
force to change their
momentum than an
object that doesn’t
bounce.
The change in
momentum is greater
when an object
bounces, therefore a
greater impulse is
needed.
To change the
momentum of an
object, an outside
unbalanced force
must be applied.
Internal forces are
always in balanced
pairs and have no
effect on the total
momentum of the
system.
If no net force or
impulse acts on
a system,
the momentum
of that system
cannot change.
This is the law of
conservation of
momentum: In the
absence of an external
force, the momentum
of a system remains
unchanged.
A 76 kg boater is at rest in
his 45 kg boat. If he steps
out of the boat at a
velocity of 2.5 m/s to the
right, what is the final
velocity of the boat?
Newton’s Second Law
states that if no net force
is exerted on a system, no
acceleration occurs. Does
it follow that no change in
momentum occurs?
Newton’s Third Law states that the
force a cannon exerts on a
cannonball is equal and opposite to
the force the cannonball exerts on
the cannon. Does it follow that the
impulse the cannon exerts on the
cannonball is equal and opposite to
the impulse the cannonball exerts
on the cannon?
The forces involved in
impulse are not constant.
The force starts small,
increases to a maximum,
then decreases.
In calculations we
use the average force.