Impulse and Momentum Unit 9

Impulse Application of force during a small amount of time Application of force during a small amount of time Applies to COLLISIONS 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:  F = ma  F = m  v/  t  F  t = m  v Left side  F  t is defined as IMPULSE Right side m  v is defined as MOMENTUM (p)

Impulse is the area under the F-t curve

 F  t = m  v Impulse = change in momentum A net force applied in a time span will cause a change of velocity or acceleration This restates Newton’s 2 nd law

Momentum by itself is not interesting, but a change of momentum is!  p = m  v How do you do this? 1.Change velocity 2.Change mass 3.Change both mass and velocity

You can also change impulse to create a change in momentum.  p = F  t How do you do this? 1.Change force 2.Change time 3.Change both force and time

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

Example 2: Decreasing momentum over a LONG time Ex. – airbags-bending knees Ex. – airbags-bending knees -landing mats-circus safety nets -yellow water barrels on roads Car accident:  p is same whether you hit a wall or a barrel Not safeF w t w = F b t b Safer Increasing IMPACT TIME reduces impact force

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

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

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

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

Summary: Impulse = F  t Impulse is area under F vs. t graph Momentum p = mv Impulse = change in momentum F  t = m  v =  p