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Impulse and Momentum

Impulse and Momentum. Unit 9. Impulse . Application of force during a small amount of time Applies to COLLISIONS Ball and bat Racket and ball Karate Car on car Airbags, yellow barrels on highway. Newton’s 2 nd Law applies to all forces: S F = ma S F = m D v/ D t S F D t = m D v

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Impulse and Momentum

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  1. Impulse and Momentum Unit 9

  2. Impulse • Application of force during a small amount of time • Applies to COLLISIONS • Ball and bat • Racket and ball • Karate • Car on car • Airbags, yellow barrels on highway

  3. Newton’s 2nd Law applies to all forces: SF = ma SF = mDv/Dt SFDt = mDv Left side SFDt is defined as IMPULSE Right side mDv is defined as MOMENTUM (p)

  4. Impulse is the area under the F-t curve

  5. SFDt = mDv • Impulse = change in momentum • A net force applied in a time span will cause a change of velocity or acceleration • This restates Newton’s 2nd law

  6. Momentum by itself is not interesting, but a change of momentum is! • Dp = mDv • How do you do this? • Change velocity • Change mass • Change both mass and velocity

  7. You can also change impulse to create a change in momentum. • Dp = FDt • How do you do this? • Change force • Change time • Change both force and time

  8. Example 1: Changing momentum • Ex. – hitting a ball -rocket engines -pushing a car -hit a ball • BIG force x BIG time = BIG D in momentum • Small force x small time = smallD in momentum

  9. Example 2: Decreasing momentum over a LONG time • Ex. – airbags -bending knees -landing mats -circus safety nets -yellow water barrels on roads Car accident: Dp is same whether you hit a wall or a barrel Not safe Fwtw = Fbtb Safer Increasing IMPACT TIME reduces impact force

  10. Example 3: Decreasing momentum in a SHORT time • Why would you want to lessen impact time to increase impact force?

  11. Example 3: Decreasing momentum in a SHORT time • Why would you want to lessen impact time to increase impact force? Discovermagazine.com

  12. What?? • Hand motion has large momentum as it approaches bricks (large Vi) • In hitting bricks, your hand experiences a large change of momentum (Vf = 0) • By bouncing hand off impact you REDUCE time of impact to a small amount • Result: Decreasing IMPACT TIME increases impact force

  13. FOLLOW THROUGH Vi = -10 m/s Vf = -5 m/s FDt = mDv F=(1kg)(5m/s)/0.01 s F=500 N BOUNCE OFF Vi = -10 m/s Vf = +10 m/s FDt = mDv F=(1kg)(20m/s)/0.01 s F=2000 N Why does bouncing create BIGGER impact force? 1 kg board 1 kg board

  14. Summary: Impulse = FDt Impulse is area under F vs. t graph Momentum p = mv Impulse = change in momentum FDt = mDv = Dp

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