Transcript Impulse and Momentum

Impulse and
Momentum
Unit 9
Impulse
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Application of force during a small
amount of time
Applies to COLLISIONS
– Ball and bat
– Racket and ball
– Karate
– Car on car
– Airbags, yellow barrels on highway
Newton’s 2nd Law applies to all forces:
SF = ma
SF = mDv/Dt
SFDt = mDv
Left side SFDt is defined as IMPULSE
Right side mDv is defined as
MOMENTUM (p)
Impulse is the area under
the F-t curve
SFDt = mDv
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Impulse = change in momentum
A net force applied in a time span will
cause a change of velocity or
acceleration
This restates Newton’s 2nd law
Momentum by itself is not
interesting, but a change of
momentum is!
Dp = mDv
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
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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
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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
Fwtw = Fbtb
Safer
Increasing IMPACT TIME reduces impact force
Example 3: Decreasing
momentum in a SHORT time
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Why would you want to lessen impact
time to increase impact force?
Example 3: Decreasing
momentum in a SHORT time
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Why would you want to lessen impact
time to increase impact force?
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What??
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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
FDt = mDv
F=(1kg)(5m/s)/0.01 s
F=500 N
FDt = mDv
F=(1kg)(20m/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
FDt = mDv = Dp