Conservation of Momentum

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Conservationof Momentum

• IDS Physics Unit 03
• M. Blachly

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Review
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Review

• If a tennis racquet applies a force of 200. N on
  a 0.15 kg tennis ball, which goes from -15 m/s to
  20. m/s as a result, how long did the collision
  last?

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Conservation of Momentum

• If there is no net external force on a system,
  then the momentum of the system is conserved.

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Conservation of Momentum

• If a system can be identified on which there is
  no net force, then momentum is conserved for that
  system.
• Internal forces do not change the total momentum
  of the system.

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Demo

• http//www.physics.umn.edu/outreach/pforce/circus/

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Example 1

• A 85 kg. boy holding a 5.0 kg. ball is at rest.
  If the boy throws the ball at 3.0 m/s to the
  left, what is his velocity after throwing the
  ball?

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Collisions

• There are two special types of collisions
• Perfectly Elastic Collisions
• Momentum is conserved
• Energy is conserved
• Completely inelastic collisions
• Momentum is conserved
• The colliding objects join and have a common
  final velocity
• Third type inelastic collisions are any
  collision that are not Perfectly Elastic or
  Completely inelastic

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Example 2

• A 1 kg. block of wood is sliding to the right
  with a speed of 10. m/s when a 100. g arrow going
  -85 m/s hits it. What is the final velocity of
  the pair after the collision?

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Example 3 Hollywood Physics?

• A 500. kg. tiger is charging some nefarious
  bad-guy at 8.0 m/s. The bad guy shoots the tiger
  with a huge, 500. gram bullet that exits the gun
  at a whopping 400. m/s. Assuming that the bullet
  lodged in the tiger, what is the final velocity
  of the beast/bullet combination?
• Disclaimer IPS, ATHS, The Magnet Academy and
  Mr. Blachly do not condone cruelty to animals nor
  do we endorse violence as a solution to the
  problem of charging tigers. This problem is used
  solely to demonstrate the impossibility of the
  projected hollywood reality.