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Unit 06b “Momentum and Impulse”

Unit 06b “Momentum and Impulse”. “Calculating Impulse: Force and Time”. Do you think the man will be hurt by the nails in the boards?. http://www.teachertube.com/viewVideo.php?video_id=21804&title=Bed_of_Nails. Impulse. Definition Equation. Definition Equation.

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Unit 06b “Momentum and Impulse”

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  1. Unit 06b “Momentum and Impulse” “Calculating Impulse: Force and Time”

  2. Do you think the man will be hurt by the nails in the boards? http://www.teachertube.com/viewVideo.php?video_id=21804&title=Bed_of_Nails

  3. Impulse Definition Equation Definition Equation Impulse is the amount of change in an objects momentum. Impulse is the product of the force applied to an object and the amount of time applied Δp = pf - pi Δp = mvf - mvi Δp = FΔ t

  4. Impulse – Momentum Theorem The change in an object’s momentum is equal to the amount of force and the amount of time applied to the object. FΔt = Δp FΔ t= mvf - mvi

  5. For a given change in momentum… The more time the object has to stop, the less force needed to stop it. The less time the object has to stop, the more force needed to stop it.

  6. For Example…. To stop an 0.5kg egg. • The change in momentum is the same no matter how you stop it. • Vi = -5m/s Vf = 0m/s (stops) • Δp = (0.5kg)(0m/s) – (0.5kg)(-5m/s) • Δp = 2.5kgm/s • BUT the force and time used to stop it can change! Δp = FΔt (Δt = 2s on a soft pillow) 2.5kgm/s = F (2s) 1.25N = F (Δt = 0.5s on the hard ground) 2.5kgm/s = F (0.5s) 5.0N = F MORE time LESS force!! LESS time MORE force!!

  7. More time to change the momentum of an object means less force applied to the object. Pillow, Fun noodle, air bag Cushion “Sinks in” “Stretches out” Bungee cord Elastics Seatbelt Suspension “bends” or “breaks” Jump off table Crumple zone Bed of nails Crumple Zone

  8. Save the Watermelon!!! • Watch the video of the man dropping a watermelon into a pool of water and onto the concrete. • Use impulse to explain why the watermelon will be safe in the water but not on the concrete? Water acts like a cushion. The watermelon sinks into the water, the watermelon MORE TIME to stop, so there is LESS FORCE on it to stop it.

  9. Watermelon Accident Facts • Mass of Watermelon = 2kg • Velocity when it hits the surface = 10m/s • Velocity after it stops = 0m/s • Time force was applied by the concrete: 0.20s • Time force was applied by the water: 2.5s

  10. Now, let’s prove it mathematically! Impulse (change in momentum)for the Watermelon

  11. Now, let’s prove it mathematically! Impulse (change in momentum)for the Watermelon m = 2kg Vi = 10m/s Vf = 0m/s Δp = mvf – mvi Δp = 2kg(0m/s) – (2kg)(10m/s) Δp = 0kgm/s – 20kgm/s Δp = -20kgm/s

  12. Force from Concrete Δt = F= ? Δp = Force from Water Δt = F= ? Δp =

  13. Force from Concrete Δt = F= ? Δp = Force from Water Δt = F= ? Δp = 0.20s 2.5s -20kgm/s -20kgm/s Δp =FΔt Δp =FΔt -20kgm/s=F(2.5s) -20kgm/s=F(0.20s) -8N = F -100N = F

  14. Car Accident Facts • Mass of an average person = 68kg • Velocity when it gets into accident= 40m/s • Velocity after it stops = 0m/s • Time force was applied by steering wheel: 0.50s • Time force was applied by seatbelt: 3.8s • Time force was applied by airbag: 7.2s

  15. Wow! Time to stop really DOES matter!How about in a car – will time to stop help save you in an accident? Impulse (change in momentum)for the Person

  16. Wow! Time to stop really DOES matter!How about in a car – will time to stop help save you in an accident? Impulse (change in momentum)for the Person m = 68kg Vi = 40m/s Vf = 0m/s Δp =? Δp = mvf – mvi Δp = 68kg(0m/s) – (68kg)(40m/s) Δp = 0kgm/s – 2720kgm/s Δp = -2720kgm/s

  17. Force from Steering Wheel Δt = F= ? Δp = Force from Seatbelt Δt = F= ? Δp = Force from Airbag Δt = F= ? Δp =

  18. Force from Steering Wheel Δt = F= ? Δp = Force from Seatbelt Δt = F= ? Δp = 0.50s 3.78s -2720kgm/s -2720kgm/s Δp =FΔt Δp =FΔt -2720kgm/s=F(3.78s) -2720kgm/s=F(0.50s) -719N = F -5440N = F Force from Airbag Δt = F= ? Δp = 7.2s Δp =FΔt -2720kgm/s=F(7.2s) -2720kgm/s -378N = F

  19. Explain how the seatbelt gives you more time to stop. Explain how the airbag gives you more time to stop. The seatbelt acts like a suspension by stretching out giving the person more time to stop. According to the impulse-momentum theorem, the more time the less force applied to stop an object. For example the steering wheel applies a big force of 5440N over a time of 0.50s, whereas the seatbelt applies a smaller 720N force over a longer time of 3.78s. The airbag acts like a cushion by sinking in to give the person more time to stop. According to the impulse-momentum theorem, the more time the less force applied to stop an object. For example the steering wheel applies a big force of 5440N over a short time of 0.50s, whereas the airbag applies a smaller 378N force over a longer time of 7.2s.

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