Impulse, Momentum and Conservation of Momentum

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Impulse, Momentum and Conservation of Momentum
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Newton Again!

• Newton observed objects colliding and realized
  that two things dictate what it takes to change
  the motion of an object.
• Mass (how much matter)
• Velocity (how fast its going in a given
  direction)

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He Called This Momentum

• p m.v

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Large Mass

• The more mass you have, the more momentum you
  will have at a given speed.

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Homer Has Momentum

• The faster Homer moves, the more momentum he has
  for his constant mass.

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Conservation of Momentum

• Lets look at what happens to momentum before,
  during and after a collision/explosion.

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Before collision
M2000 kg, v 5 m/s
M 500 kg, v 0
After collision The system has the same total
momentum.
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Impulse J

• By rewriting his own 2nd law, Newton defined
  impulse
• F m . a m .
• F . t m . ?v
• J F . t m?v ?p change in momentum

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So Force . time?Momentum

• Lets say Mini-me was trying to stop Dr. Evil from
  running at him.
• Mimi-mis relatively small force on Dr. Evil
  would have to be exerted over a long period of
  time to change the momentum of Dr. Evil.

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• So the safest way to stop an object is to stretch
  out the time it takes to stop things so you could
  use the smallest possible force. Thats why if
  something is cushioned and soft it is less likely
  to break.

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Momentum is Conserved
TZ student defend school with potato
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Conservation of Momentum

• Newtons Third Law
• The potato goes one way and the gun recoils in
  the opposite direction.
• The gun exerts a force on the potato and the
  potato exerts an equal but opposite force on the
  gun, recoil.

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pGun/Girl pPotato

• These forces produce equal but opposite changes
  in momentum.
• Since the girl is attached to the gun, the
  combination of the girl /gun mass is much greater
  than the mass of the potato.
• The girl and the gun recoil at a velocity much
  smaller than the potato.

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If pGunPerson pPotato Then

• Mgun/person.vgun/personmpotatoe.vpotato
• If the mass on the left of the equation is large
  compared to the right, then the velocity on the
  right must be large compared to the left if they
  are to be equal.

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Action/Reaction

• Conservation of momentum is just a consequence of
  Newtons third Law