Physics 2211: Lecture 18

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Physics 2211 Lecture 18

• More on Friction
• Drag

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Friction force vs. Applied Force
f
?SN
?KN
f FA
FA
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A 50 kg steel file cabinet sits in the back of a
dump truck. The trucks bed, also made of steel,
is slowly tilted. What is the size of the static
friction force on the cabinet when the bed is
tilted 20 degrees? At what angle will the file
cabinet begin to slide?
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Static Friction on An Inclined Plane
N
f
?
mg
The applied force is the component of the
weight parallel To the plane.
Therefore, the static friction force will vary
with angle.
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N
y
f
x
?
mg
Assess!
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N
y
f
x
?
mg
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A horizontal force P 12 N pushes on a M 6 kg
mass that slides on a frictionless surface. Due
to friction, a smaller mass (m 4 kg) stays
fixed with respect to the larger mass. What is
the force that m exerts on M?
1 4.8 N to the right 2 4.8 N to the
left 3 6.1 N to the right
4 6.1 N to the left 5 7.2 N to the
left 6 7.2 N to the left
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Drag
A force experienced by a body that moves through
air.
Experiment shows that to a good approximation,
Unit vector that points in the direction of the
velocity of the body
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Throw a ball upward vertically
Drag decreases as the ball slows down
Drag increases as the ball speeds up slows
Drag opposes the weight as it falls
Drag adds to the weight as it rises
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Terminal velocity
At the terminal velocity, the drag force
balances the force of gravity.
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Drop a particle from rest (v 0)
As the speed (and thus drag) increases, the
slope decreases
Slope approaches zero as v approaches terminal
velocity
Without drag, v - at
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Words to the Wise
Study the Worked Examples In Section 5.6 of
Knight
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Recap
More on Friction Drag
Next Lecture
Motion in a Plane Knight Chapter 6