aim: how can we explain momentum and impulse? do now: which is easier to do: stop a skateboard...
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Aim: How can we Aim: How can we explain momentum and explain momentum and
impulse?impulse?
Do Now:
Which is easier to do:
Stop a skateboard traveling at 5 m/s or stop a car traveling at 5 m/s?
Why?
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Which is easier to do:
Stop a bullet fired from a gun or stop a bullet that is thrown at you?
Why?
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Momentum (p)Momentum (p)•“Mass in motion”•Momentum = mass * velocity•Or p = mv•Vector quantity
What about the units?•Units are kg·m/s
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What is the momentum of What is the momentum of a 60 kg halfback moving a 60 kg halfback moving 9 m/s, eastward?9 m/s, eastward?
p = mvp = (60 kg)(9 m/s)p = 540 kg·m/s eastward
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If a 1 kg ball bounces off of a wall with the same velocity as shown below, is there a change in momentum?
Yes because there has been a change in direction
Momentum is a vector quantity
v = 10 m/s
v = 10 m/s
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So how do we define a change in momentum?
Δp = pf – pi
Δp = mvf – mvi
Δp = m(vf – vi)
Δp = mΔv
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Calculate the change in momentum of the bouncing ball
Δp = mΔv
Δp = (1 kg)(-10 m/s – 10 m/s)
Δp = (1 kg)(-20 m/s)
Δp = -20 kg·m/s
The negative sign indicates a change in direction
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Why do you follow through when:
•Swinging a baseball bat
•Swinging a tennis racket
•Swinging a golf club
•Kicking a football
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Impulse (J)Impulse (J)• A force must act on an object for
a time in order to change its velocity
• Impulse (J) = Force * time• Or J = Ft• Vector quantity
What about the units?• The units are N·s
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Calculate the impulse on a baseball being hit by a baseball bat with a force of 1200 N over 0.02 s
J = Ft
J = (1200 N)(0.02 s)
J = 24 N·s
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Egg Demo
Why doesn’t the egg break when it hits the bed sheet?
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We know:We know:
t
vmF
Doesn’t J = Ft?
Doesn’t Δp = mΔv?
So J = Δp
Impulse is a change in momentum!!
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mΔv is a constant•mass has not changed•initial and final velocities have not changed
Time to slow down increasedTherefore force has to decreaseHence, the egg does not break!
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Real-World ApplicationsReal-World Applications•Baseball and tennis player’s ‘following through’
•Boxer’s ‘riding the punch’•Airbags•Padded dashboards
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What if force and time are constant but the mass changes?
Astroblaster Demo
If mass decreases, velocity must increase!
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A car with m=725 kg is moving at 32 m/s to the east. A car with m=725 kg is moving at 32 m/s to the east. The driver applies the brakes for 2 s. An average force The driver applies the brakes for 2 s. An average force of 5.0 x 10of 5.0 x 1033 N is exerted on the car. What is the change N is exerted on the car. What is the change in momentum? in momentum?
Δp = Ft
Δp = (-5 x 103 N)(2 s)
Δp = -1 x 104 N
What is the impulse on the car?What is the impulse on the car?
J = Δp
J = -1 x 104 N
What is the car’s final velocity?What is the car’s final velocity?
Δp = mΔv
-1 x 104 N = (725 kg)(vf – 32 m/s)
-1 x 104 = 725vf – 23,200
vf = 18.2 m/s
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An impulse of 30.0 N·s is applied to a 5.00 kg mass. If the mass had a speed of 100 m/s before the impulse, what is its speed after the impulse?
J = mΔv
30 N·s = (5 kg)(vf – 100 m/s)
30 = 5vf – 500
vf = 106 m/s
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A car with a mass of 1.0 x 103 kg is moving with a speed of 1.4 x 102 m/s. What is the impulse required to bring the car to rest?
J = mΔv
J = (1 x 103 kg)(0 m/s – 1.4 x 102 m/s)
J = -1.4 x 105 N·s