Transcript Momentum Impulse II

What We Know About
Momentum…
• The product of an object’s mass and
velocity.
• ρ = mv
– Units are kg x m/s
– Why isn’t m or M used for the symbol of
momentum?
Very Important: In order for an object to
have momentum it must…………..
..........................be moving!
If you are asked to give an example of
something with a lot of momentum and
your response is a bullet, or a mack truck,
or an NFL linebacker, you are WRONG!
Changing momentum can be done
two ways.
• A momentum change
can take place over a
long period of time:
– FΔt
• Or over a short period
of time:
– FΔt
Example: Changing the
Momentum of a Thrown Egg
• Egg
mass = .056 kg
velocity= 9 m/s
p = ___________
Over a Long Period of Time:
t = 0.9 s
p = .504 kg X m/s
F∆t = m∆v
F(0.9s) = .504 kg x m/s
F = .56 N
Therefore, the egg does not break
Over a Short Period of Time:
p = .504 kg x m/s
F∆t = m∆v
F(0.09s) = .504 kg x m/s
F = 5.6 N
Therefore, the egg breaks
t = 0.09 s
In Summary:
• For an object with a given amount of
momentum
t ↑ then F↓
t ↓ then F↑
Other Examples:
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Egg toss
Padded dashboard in the car
Air bags in the automobile
Baseball catcher
Jump from the table top (flex the knees)
Boxers “rolling with the punches”
Bungee Jumping
Stunt person off a building and into an air mat
Collisions
• There are two types
of collisions that we
will discuss in class.
In one, the objects
bounce off of each
other and in the
second, the two
collide and then stick
together.
Two Types of Collisions
• Elastic- two objects collide and return to their original shapes
with no change in total kinetic energy.
– “bounce” away from each other
– Billiard balls, glass balls colliding are nearly elastic
– Key Words: bounce, hit and recoil
• Inelastic- occurs when two objects collide and move together
as one mass, like two railroad cars coupling.
– momentum is still conserved, but there is some energy
“lost”
– Key Words: stick together, couple, become embedded
The Collision Formula, also known as the
Law of Conservation of Linear Momentum
• m1v1 + m2v2 = m1v1΄ + m2v2΄
If they stick together, then v1΄ and v2΄ are the
same making the formula look like this:
m1v1 + m2v2 = ( m1 + m2 ) v ΄
Or,
Mobefore = Moafter
The Law of Conservation of
Momentum
• In the absence of an outside force, the
momentum of a system remains
unchanged. In other words, the
momentum before equals the momentum
after.