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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Impulse

• By rewriting his own 2nd law, Newton defined
  Impulse
• Fm.am.
• F.tm.?v IMPULSE J
• JF.t m.?v?pchange in momentum

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If a 5 N force pushes a 2 kg object for 3
seconds, how much will the momentum change?

How much is the impulse? How much will the speed
change? How much is the acceleration?
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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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If pGun/Emily pPotato Then

• Mgun/Emily.vgun/Emilympotatoe.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

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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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Momentum is Conserved
TZ student defends school with potato gun
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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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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 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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Action/Reaction

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