Momentum and Impulse

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Momentum and Impulse
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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
• C. The same

Bouncing Ball
Sticky Ball
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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

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Example stopping car
m 1500 kg
How long does it take the car to stop?
Vo40 m/s
µs 0.6
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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
after
before
Vo 0m/s
Vo 5m/s
Two blocks collide and stick together, what is
their final velocity?
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Example collision
after
before
Vo 2m/s
Vo 5m/s
Two blocks collide and stick together, what is
their final velocity?
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Example cannon (explosion)
mc 200kg
What is the velocity of the cannon?
mb 5 kg v 300 m/s
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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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