Momentum and Impulse
Key Ideas

Last Time: Work-Energy
S F = m a
multiply both sides by d
S F d= m a d (note a d = ½ Dv2)
S F d= ½ m Dv2
S W = DKE
Define Work and Kinetic Energy

This Time: Impulse-Momentum
S F = m a
multiply both sides by Dt
S F Dt= m a Dt (note a Dt = Dv)
S F Dt= m Dv
S I = Dp
Define Impulse and Momentum
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Demo/Example
Two identical balls are dropped from the same height onto the
floor. In each case they have velocity v downward just before
hitting the floor. In case 1 the ball bounces back up, and in
case 2 the ball sticks to the floor without bouncing. In which
case is the magnitude of the impulse given to the ball by the
floor the biggest?
A. Case 1
B. Case 2
Bouncing Ball
Sticky Ball
C. The same
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Question
In both cases of the above question, the
direction of the impulse given to the ball by the
floor is the same. What is this direction?
A. Upward
B. Downward
time
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m = 1500 kg
Example: stopping car
Vo=40 m/s
µs = 0.6
How long does it take the car to stop?
Momentum is Conserved
 Momentum
is “Conserved” meaning
it can not be created nor destroyed
Can be transferred
 Total
Momentum does not change
with time.
This
is a BIG deal!
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Example: collision
“before” Vo = 0m/s
Vo = 5m/s
M1=2kg
M2=3kg
“after”
M1=2kg
M2=3kg
Two blocks collide and stick together, what is their final velocity?
Example: collision
“before” Vo = 2m/s
Vo = 5m/s
M1=2kg
M2=3kg
“after”
M1=2kg
M2=3kg
Two blocks collide and stick together, what is their final velocity?
Example: cannon (explosion)
mc = 200kg
mb = 5 kg
v = 300 m/s
What is the velocity of
the cannon?
Impulse and Momentum
Summary

Collisions and Explosions
Draw “before”, “after”
Define system so that Fext = 0
Set up axes
Compute Ptotal “before”
Compute Ptotal “after”
Set them equal to each other
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Summary
Impulse I = FDt
» Gives change in momentum I = Dp
Momentum p = mv
» Momentum is VECTOR
» Momentum is conserved (when SF = 0)
 S mvinitial = S mvfinal
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