Chapter-7 Momentum and Impulse

Impulse:Change in MomentumCan momentum change?Change in Momentump = m vChange in momentum = mass x change in velocity

(Units) kgm/s = kg x m/sTo stop an object, we have to apply a force to the object over a period of time.

This is called IMPULSE.

J = F tImpulse = Force x change in time

Ns = N x sHow hard is it to stop a moving object?How do momentum and impulse relate to force?Combine these two formulas and simplify: F = ma a = v/tFt = mvSO J = pF = p/ t Short FormUseful FormWork through the substitutions in class. Substitute change in V/ t for acceleration, then move the / t over to the Force side.4Why 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?

Ft= mv

Hint 1: In both cases, m and v are the same. Hint 2: If t 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.

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.JPG6Car Crash (cont.)Answer is It Does Not Matter!

Look at Ft= mv

In both situations, t, m, and v are the same!

The time it takes you to stop depends on your car, m is the mass of your car, and v depends on how fast you were initially traveling.

Minimize the ForceApply the force for a long time.

Ft

Catching a BallBungee JumpingAirbagsMaximize the ForceApply the force for a short time.

Ft

BuntingKarate

Boxing

How does a helmet reduce force on your head?

Its all about time!!!Helmet and bare head stop a 90 mph fastball, mv is the same for both situations

F= mDv Small t for head without helmetBig ForceF= mDvBig t for head with helmetSmall ForcettReal World ApplicationsSafe CollisionsCollisions that take a long time are better, because they allow for smaller forces.

How do we increase the time of a collision?Safety in Sports

All these products increase the time of the collision.Gmax Device

5 kg mass is dropped onto an athletic surfaceHeight of the bounce is measuredGmax value is determinedThe bigger the bounce the harder the surfaceDifferent surface has different allowable Gmax values basketball court > football fieldSummaryLarge amounts of time in a collision, results in a smaller force.

Motion can be described by position, velocity and momentum.Changes in motion can be described by acceleration and changes in momentum.Changes in motion are caused by forces.Newtons Laws help us predict changes in motion.Collisions are about momentum, and Newtons 2nd and 3rd Laws.Watch Out!You drive around a curve in a narrow one-way street at 30 mph when you see an identical car heading straight toward you at 30 mph. You have two options: hit the car head-on or swerve into a massive concrete wall (also head-on). What should you do?1) hit the other car2) hit the wall3) makes no difference 4) call your physics prof!!5) get insurance! 16Answer: cIn both cases your momentum will decrease to zero in the collision. Given that the time Dt of the collision is the same, then the force exerted on YOU will be the same!! If a truck is approaching at 30 mph, then youd be better off hitting the wall in that case. On the other hand, if its only a mosquito, well, youd be better off running him down... Watch Out!You drive around a curve in a narrow one-way street at 30 mph when you see an identical car heading straight toward you at 30 mph. You have two options: hit the car head-on or swerve into a massive concrete wall (also head-on). What should you do?1) hit the other car2) hit the wall3) makes no difference 4) call your physics prof!!5) get insurance! 17Two BoxesF F lightheavy1) the heavier one 2) the lighter one3) both the same

Two boxes, one heavier than the other, are initially at rest on a horizontal frictionless surface. The same constant force F acts on each one for exactly 1 second. Which box has more momentum after the force acts ?18Answer: cF F lightheavyavDtDpF=We know:so impulse Dp = Fav Dt.In this case F and Dt are the same for both boxes!Both boxes will have the same final momentum.,Two Boxes1) the heavier one 2) the lighter one3) both the same

Two boxes, one heavier than the other, are initially at rest on a horizontal frictionless surface. The same constant force F acts on each one for exactly 1 second. Which box has more momentum after the force acts ?19In the previous question, which box has the larger velocity after the force acts? 1) the heavier one2) the lighter one3) both the sameTwo Boxes20Answer: bThe force is related to the acceleration by Newtons 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?In the previous question, which box has the larger velocity after the force acts? 1) the heavier one2) the lighter one3) both the sameTwo Boxes21Collision Course1) the car2) the truck3) they both have the same momentum change4) cant tell without knowing the final velocitiesA small car and a large truck collide head-on and stick together. Which one has the larger magnitude of momentum change?22Answer: cBecause the total momentum of the system is conserved, that means that Dp = 0 for the car and truck combined. Therefore, Dpcar must be equal and opposite to that of the truck (Dptruck) in order for the total momentum change to be zero. Note that this conclusion also follows from Newtons Third Law.Follow-up: Which one feels the larger acceleration?Collision Course1) the car2) the truck3) they both have the same momentum change4) cant tell without knowing the final velocitiesA small car and a large truck collide head-on and stick together. Which one has the larger magnitude of momentum change?23Shut the Door!1) the superball2) the blob of clay3) it doesnt matterthey will be equally effective4) you are just too lazy to throw anythingYou 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?24Answer: aThe 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 with it, so its Dp is less than that of the superball, and therefore it imparts less Dp to the door.Shut the Door!1) the superball2) the blob of clay3) it doesnt matterthey will be equally effective4) you are just too lazy to throw anythingYou 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