Momentum
MomentumWould 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 = mass x velocityMomentum is
inertia in motion.Example: What is the momentum of a 10 kg bowling
ball moving at 20 m/s ?mom = m x vmom = 10 kg x 20 m/smom = 200 kg
m/sWhat would you have to change to change momentum?To change
momentum, you can change mass or velocity. These one mass problems
will always involve changing 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.Many sports
involve a change in momentum.Baseball
Golf
Soccer
Impulse is a physics term that relates a force and the amount of
time it is applied.force x time = impulseExample: 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 sImpulse = force x time50 N x 0.2 s = 10 Ns
Try problems 1-12It is impulse that causes a change in momentum.
is a symbol used to indicate a change in something.Ft = mom A
change in anything is the final value the initial value. Example:
What is the change in your bank account if you start with $100 and
end up with $500?$400
What is the change in momentum if a 1000 kg car accelerates from
20 m/s to 50 m/s?30,000 kg m/sm = 1000 kgvi = 20 m/s (initial
speed)vf = 50 m/s (final speed)final momentum = mvf = 1000 kg x 50
m/s final momentum = 50000 kg m/sinital momentum = mvi = 1000 kg x
20 m/s initial momentum = 20000 kg m/s
mom = final momentum initial momentum mom = 50000 kg m/s 20000
kg m/s = 30,000 kg m/sA 5 kg mouse is moving at 10 m/s. If he comes
to a stop, what was his change in momentum?
m = 5 kgvi = 10 m/svf = 0 m/sfinal momentum = 0 kg m/sinitial
momentum = 50 kg m/s mom = 0 kg m/s 50 kg m/s mom = - 50 kg m/sThe
negativeindicates a loss of momentum.
Since impulse causes a change in momentumFt = momExample: A 20
kg object starts at rest. In 5 s, it has a speed of 100 m/s. How
much force was used?400 NSolutionm = 20 kgvi = 0 m/svf = 100 m/st =
5 sF = ?
Ft = mvf - mvi
F(5) = (20)(100) (20)(0)F(5) = 2000 - 0F(5) = 2000
F = 2000/5F = 400 NTry Problems 13-16Give 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 NThe 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/sTry problems 17-22
Conservation of MomentumPhysics 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 collisionsThe recoil of a
gun is a good example of conservation of momentum. Initial speed of
both objects is zero.
http://www.youtube.com/watch?v=GDRE2gvRHn8
Gun RecoilSpud Gun - Recoil
Fire Extinguisher
Recoilhttp://www.youtube.com/watch?v=K8YmPjt4_BU
Most Collisions are Elastic
Inelastic CollisionsIn an inelastic collision, the objects
involved in the collision stick together after they collide. An
Example
Conservation Of MomentumFor CollisionsThe 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?Momentum before
= 5 kg x 10 m/s Momentum before = 50 kg m/sMomentum after = (10 kg)
v mom before = mom after 50 kg m/s = (10kg)v50 kg m/s
(10kg)v------------- = -----------10 kg (10kg)V = 5 m/sInelastic
ExampleA 1000 kg car moving at 20 m/s strikes another 1000 kg car
at rest. Afterwards, they stick together. What is their
speed?momentum before = 1000 kg x 20 m/s
momentum before = 20,000 kg m/smom after = (1000 kg+1000
kg)vSince the cars stick together, the mass after the collision is
the total of the two masses.mom before = mom after20,000 kg m/s =
(2000kg)vV = 10 m/s
