Would you rather be hit by a
tennis ball or a bowling ball?
To answer the question, you would
want to consider mass but also
velocity. That is momentum.
Momentum is inertia in motion.
momentum = mass x velocity
Ex: What is the momentum of a
10 kg bowling ball moving at 20
m/s ?
p=mxv
p = 10 kg x 20 m/s
p = 200 kg m/s
What would you have to change
to change momentum?
To change momentum, you can
change mass or velocity.
What is the name for a change in
velocity?
Acceleration.
What does it take to cause an
acceleration?
A force is needed to cause an
acceleration.
The longer a force is
applied, the greater the
change in momentum.
Baseball
Golf
Soccer
Many sports
involve a
change in
momentum.
Impulse is a physics term that
relates a force and the amount
of time it is applied.
force x time = impulse
Example: What impulse is applied
to a baseball if the batter uses a
50 N force for 0.2 seconds?
F = 50 N
t = 0.2 s
Impulse = force x time
50 N x 0.2 s = 10 Ns
It is impulse that causes a
change in momentum.
So Ft = ∆ p
Ft = pf - pi
Ft = mvf - mvi
What is the change in
momentum if a 1000 kg car
accelerates from 20 m/s to
50 m/s? 30,000 kg m/s
A 5 kg mouse is moving at 10 m/s.
If he comes to a stop, what was his
change in momentum?
The negative
m = 5 kg
vi = 10 m/s
vf = 0 m/s
final momentum = 0 kg m/s
initial momentum = 50 kg m/s
∆ p = 0 kg m/s – 50 kg m/s
∆ p = - 50 kg m/s
indicates a
loss of
momentum.
Example: A 20 kg object starts at
rest. In 5 s, it has a speed of 100
m/s. How much force was used?
400 N
Give examples of a moving object
coming to a rest.
How could you minimize the
force felt by these objects?
Give examples of ways in
which the time for an object to
stop is increased.
Airbags
A 800 kg car is moving at 30 m/s. The
driver slams on the breaks. It takes the
car 3 s to stop. What force did the
brakes supply? - 8000 N
The negative indicates the force is in
the opposite direction of the motion.
If the driver of the car above eased on
the breaks for 30 s to come to a stop,
how much force did the breaks supply?
- 800 N
Example: A 40 N force is
applied to a 5 kg object for 10 s.
If the object was initially
moving at 20 m/s, what is its
new speed?
100 m/s
Conservation of Momentum
Physics Definition: In the absence of
an external force, the momentum of a
system remains unchanged.
A system can be one or more objects.
Example of internal forces: explosions
collisions
The recoil of a gun is a good
example of conservation of
momentum.
Gun Recoil
Another Example
On A Larger Scale
Inelastic Collisions
In an inelastic collision, the objects
involved in the collision stick
together after they collide.
An Example
Conservation Of Momentum
For Collisions
The momentum before a collision
is equal to the momentum after a
collision.
momentum before = momentum after
A 5 kg object moving at 10 m/s
collides with a 10 kg object at rest.
After the collision, the 5 kg object
stops moving. What is the speed of
the 10 kg object?
Inelastic Example
A 1000 kg car moving at 20 m/s
strikes another 1000 kg car at rest.
Afterwards, they stick together.
What is their speed?
V = 10 m/s
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6/23 Graduation
A 0.50 kg object is at rest. A 3.00 N
force to the right acts on the object
during a time interval of 1.50 s.
What is the new speed of the
object?
9 m/s
A 2500 kg car travelling to the
north is slowed down uniformly
from an initial velocity of 20.0 m/s
by a 6250 N braking force acting
opposite the car’s motion. What
is the car’s velocity after 2.50 s?
13.8 m/s
A 6.00 g bullet moving at 375 m/s
is fired into a stationary 5.50 kg
wooden block which is at rest on
a flat surface. After moving
7.00 cm, the block and the
embedded bullet come to a rest.
What is the coefficient of friction?
0.12
Ballistic Pendulum
A 7.50 g bullet moving at 420 m/s
is fired into an 8.00 kg ballistic
pendulum. How high does the
pendulum rise?
0.776 cm
An elastic collision is one where
both the momentum and the kinetic
energies are conserved.
A 4.0 kg bowling ball sliding to the right
at 8.0 m/s has an elastic head-on
collision with another 4.0 kg bowling
ball initially at rest. The first ball stops
after the collision. Verify that kinetic
energy is conserved.
Calculate the recoil velocity of a
4.0 kg rifle that shoots a 0.050 kg
bullet at a speed of 280 m/s.
-3.5 m/s
A ball of mass 0.440 kg moving with
a speed of 4.50 m/s collides head
on with a 0.220 kg ball at rest. If the
collision is perfectly elastic, what will
be the speed and direction of each
ball after the collision?
1.50 m/s and
6.00 m/s
A 1500 kg car traveling east with a
speed of 25.0 m/s collides
inelastically at an intersection with a
2500 kg van traveling north at speed
of 20.0 m/s. Find their velocity after
the crash.
15.6 m/s @
0
53.1 N
of E