Impulse and Momentum Unit 9 Impulse Application of

Impulse Application of force during a small amount of time n Applies to COLLISIONS

Newton’s 2 nd Law applies to all forces: SF = ma SF = m.

SFDt = m. Dv Impulse = change in momentum n A net force applied

Momentum by itself is not interesting, but a change of momentum is! Dp =

You can also change impulse to create a change in momentum. Dp = FDt

Example 1: Changing momentum n n n Ex. – hitting a ball -pushing a

Example 2: Decreasing momentum over a LONG time n Ex. – airbags -bending knees

Example 3: Decreasing momentum in a SHORT time n Why would you want to

What? ? n n Hand motion has large momentum as it approaches bricks (large

Why does bouncing create BIGGER impact force? FOLLOW THROUGH Vi = -10 m/s 1

Summary: Impulse = FDt Impulse is area under F vs. t graph Momentum p

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Impulse and Momentum Unit 9

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

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

Impulse is the area under the F-t curve

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

Momentum by itself is not interesting, but a change of momentum is! Dp = m. Dv 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. Dp = FDt How do you do this? 1. Change force 2. Change time 3. Change both force and time

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

Example 2: Decreasing momentum over a LONG time n 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 F wt w = F b t b Safer Increasing IMPACT TIME reduces impact force

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

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

What? ? n n 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

Why does bouncing create BIGGER impact force? FOLLOW THROUGH Vi = -10 m/s 1 kg board BOUNCE OFF Vi = -10 m/s 1 kg board Vf = -5 m/s Vf = +10 m/s FD t = m D v F=(1 kg)(5 m/s)/0. 01 s F=500 N FD t = m D v F=(1 kg)(20 m/s)/0. 01 s F=2000 N

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