Physics 2211: Lecture 092404 Friday Todays Agenda

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Physics 2211 Lecture 09/24/04 FridayTodays
Agenda

• 1) Model of friction
• 2) Drag

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A 50kg stell file cabinet is in a 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 20deg. At what angle will the file cabinet
begin to slide?
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Static Friction

• We consider F an inclined plane.
• In this case, the force provided by friction will
  depend on the angle of the plane

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Static Friction...

• The force provided by friction, fF , depends on

fF
ma 0 (filing cabinet is not moving)
mg sin ????ff ???
N
?
(Newtons 2nd Law along x-axis)
mg
?
ffmg sin ??50kg9.8m/s2sin20168N
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• increasing the ramp angle just until the block
  slides

mg sin ????ff????
In this case
?ff????SN ? ??Smg cos ?M
?SN
mg sin ?M????Smg cos ?M????
N
mg
??M
?S???tan ?M?
?
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Question 1 (skip this do it later using 3rd
Newton law)

• A box of mass m1 1.5 kg is being pulled by a
  horizontal string having tension T 90 N. It
  slides with friction (mk 0.51) on top of a
  second box having mass m2 3 kg, which in turn
  slides on a frictionless floor.
• What is the acceleration of the second box ?
• (1) a 0 m/s2 (2) a 2.5 m/s2 (3) a 3.0
  m/s2

slides with friction (mk0.51 )
T
m1

a ?
m2
slides without friction
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Question 2

• A 1000 kg car traveling at a speed of 40m/s
  skids to a halt on wet concrete where mk0.6. How
  long are the skid-marks?
• 1)25m 2)45m 3)120m 4)135m
• 5)230m

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Causes of friction
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Problem
v0
a
An object is moving up an inclined plane with an
initial speed equal to v0. The angle of the plane
is larger than tan-1mS, where mS is the static
friction coefficient. As a result, the objects
returns down to the bottom of the plane with
speed equal to v0/2. Find the coefficient of
kinetic friction.
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Problem
v0
a
An object is moving up an inclined plane with an
initial speed equal to v0. The angle of the plane
is larger than tan-1mS, where mS is the static
friction coefficient. As a result, the objects
returns down to the bottom of the plane with
speed equal to v0/2. Find the coefficient of
kinetic friction.
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Drag
Drag force is opposite in direction to velocity
Increases in magnitude with speed
Direction opposite the motion
A cross-section areavspeed
Has units of of kg/m3
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Cross-sectional areas
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Example

• The profile of a typical 1500kg car, as seen from
  the front, is 1.6m wide and 1.4m high. At what
  speed does the magnitude of the drag equal the
  magnitude of the rolling friction?

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answer
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Ball moving vertically
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Terminal speed

• Dw

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Question 3

• What is a terminal speed for an 80kg skier going
  down a 40deg snow-covered slope on wooden skis?
  Assume that the skier is 1.8m tall and 0.4m wide

1) 40m/s 2)51m/s 3)95m/s 4)66m/s 5)19m/s
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