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Impulse: Change in Momentum

Impulse: Change in Momentum. Change in Momentum. ∆p = m ∆v Change in momentum = mass x change in velocity (Units) kg•m /s = kg x m/s. Can momentum change?. How hard is it to stop a moving object?. J = F ∆t Impulse = Force x change in time

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Impulse: Change in Momentum

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  1. Impulse: Change in Momentum

  2. Change in Momentum ∆p = m ∆v Change in momentum = mass x change in velocity (Units) kg•m/s = kg x m/s Can momentum change?

  3. How hard is it to stop a moving object? J = F ∆t Impulse = Force x change in time N•s = N x s To stop an object, we have to apply a force to the object over a period of time. This is called IMPULSE.

  4. How do momentum and impulse relate to force? Combine these two formulas and simplify: F = ma a = ∆v/∆t Useful Form F∆t = m∆v SO…… J = ∆p F = ∆p/∆t Short Form

  5. Why does an egg break or not break? An egg dropped on a tile floor breaks, but an egg dropped on a pillow does not. Why? FΔt= mΔv Hint 1: In both cases, m and Δv are the same. Hint 2: If Δt goes up, what happens to F, the force? Answer: When dropped on a pillow, the egg starts to slow down as soon as it touches it. A pillow increases the time the egg takes to stop.

  6. Car Crash Would you rather be in a head on collision with an identical car, traveling at the same speed as you, or a brick wall? Assume in both situations you come to a complete stop. http://techdigestuk.typepad.com/photos/uncategorized/car_crash.JPG

  7. Car Crash (cont.) Answer is… It Does Not Matter! Look at FΔt= mΔv In both situations, Δt, m, and Δv are the same! The time it takes you to stop depends on your car, m is the mass of your car, and Δv depends on how fast you were initially traveling.

  8. Minimize the Force Catching a Ball F∆t Airbags Bungee Jumping Apply the force for a long time.

  9. Maximize the Force F∆t Boxing Bunting Karate Apply the force for a short time.

  10. Small “t” for head without helmet Big Force Big “t” for head with helmet Small Force How does a helmet reduce force on your head? Helmet and bare head stop a 90 mph fastball, m∆v is the same for both situations F= mDv It’s all about time!!! t F= mDv t

  11. Real World Applications

  12. Safe Collisions Collisions that take a long time are better, because they allow for smaller forces. How do we increase the time of a collision?

  13. Safety in Sports All these products increase the time of the collision.

  14. Gmax Device 5 kg mass is dropped onto an athletic surface Height of the bounce is measured Gmax value is determined The bigger the bounce the harder the surface Different surface has different allowable Gmaxvalues basketball court > football field

  15. Summary……… Collisions are about momentum, and Newton’s 2nd and 3rdLaws. Changes in motion can be described by acceleration and changes in momentum. Newton’s Laws help us predict changes in motion. Motion can be described by position, velocity and momentum. Changes in motion are caused by forces. Large amounts of time in a collision, results in a smaller force.

  16. Watch Out! 1) hit the other car 2) hit the wall 3) makes no difference 4) call your physics prof!! 5) get insurance! You drive around a curve in a narrow one-way street at 30 mph when you see an identicalcar heading straight toward you at 30 mph. You have two options: hit the car head-onorswerve into a massive concrete wall(also head-on). What should you do?

  17. Watch Out! You drive around a curve in a narrow one-way street at 30 mph when you see an identicalcar heading straight toward you at 30 mph. You have two options: hit the car head-onorswerve into a massive concrete wall(also head-on). What should you do? 1) hit the other car 2) hit the wall 3) makes no difference 4) call your physics prof!! 5) get insurance! Inbothcases your momentum will decrease to zero in the collision. Given that thetime Dtof the collision is the same, then theforce exerted on YOUwill be the same!! If atruckis approaching at 30 mph, then you’d be better offhitting the wallin that case. On the other hand, if it’s only amosquito, well, you’d be better offrunning him down...

  18. F F heavy light Two Boxes 1) the heavier one 2) the lighter one 3) both the same Two boxes, one heavier than the other, are initially at rest on a horizontal frictionless surface. The same constant force Facts on each one for exactly 1 second. Which box has more momentum after the force acts ?

  19. Dp = F Dt av F F heavy light Two Boxes Two boxes, one heavier than the other, are initially at rest on a horizontal frictionless surface. The same constant force Facts on each one for exactly 1 second. Which box has more momentum after the force acts ? 1) the heavier one 2) the lighter one 3) both the same , We know: so impulseDp = Fav Dt. In this caseFand Dtare thesamefor both boxes! Both boxes will have thesame final momentum.

  20. Two Boxes In the previous question, which box has the larger velocity after the force acts? 1) the heavier one 2) the lighter one 3) both the same

  21. Two Boxes In the previous question, which box has the larger velocity after the force acts? 1) the heavier one 2) the lighter one 3) both the same The force is related to the acceleration by Newton’s Second Law (F = ma). The lighter box therefore has the greater acceleration and will reach a higher speed after the 1-second time interval. Follow-up: Which box has gone a larger distance after the force acts? Follow-up: Which box has gained more KE after the force acts?

  22. Collision Course 1) the car 2) the truck 3) they both have the same momentum change 4) can’t tell without knowing the final velocities A small car and a large truckcollide head-on and stick together. Which one has the larger magnitude of momentum change?

  23. Collision Course A small car and a large truckcollide head-on and stick together. Which one has the larger magnitude of momentum change? 1) the car 2) the truck 3) they both have the same momentum change 4) can’t tell without knowing the final velocities Because the total momentum of the system is conserved, that means thatDp= 0for thecar and truck combined. Therefore,Dpcarmust beequal and oppositeto that of the truck(–Dptruck)in order for the total momentum change to be zero. Note that this conclusion also follows from Newton’s Third Law. Follow-up: Which one feels the larger acceleration?

  24. Shut the Door! 1) the superball 2) the blob of clay 3) it doesn’t matter—they will be equally effective 4) you are just too lazy to throw anything You are lying in bed and you want to shut your bedroom door. You have a superball and a blob of clay (both with the same mass) sitting next to you. Which one would be more effective to throw at your door to close it?

  25. Shut the Door! You are lying in bed and you want to shut your bedroom door. You have a superball and a blob of clay (both with the same mass) sitting next to you. Which one would be more effective to throw at your door to close it? 1) the superball 2) the blob of clay 3) it doesn’t matter—they will be equally effective 4) you are just too lazy to throw anything The superball bounces off the door with almost no loss of speed,so its Dp (and that of the door) is 2mv. The clay sticks to the door and continues to move along withit,so its Dp is less than that of the superball, and therefore it imparts less Dp to the door.

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