"Deal with December Now" Workshop

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"Deal with December Now" Workshop

• Help with preparation for the exams
• More information on the workshop can be found at
• www.sciteam.ubc.ca

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Friction

• Kinetic friction
• Static friction
• Friction force does not depend on the contact
  area!
• Demo with the block pulled by a spring scale

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Coefficients of friction

• Static
• S max(fs)/n
• fs ? ? S n
• Kinetic
• K fk /n
• fk ? K n
• ? S gt ? K

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Coefficients of friction

• Static
• S max(fs)/n
• fs ? ? S n
• Kinetic
• K fk /n
• fk ? K n
• ? S gt ? K

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DemoDishes on a dishcloth
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Wheels on the surface

• Rolling friction small friction force.
  Corresponding coefficient of friction of the
  order of 0.01.

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Example Braking car

• A car of mass m 1000 kg driven at 50 km/h
  suddenly brakes locking the wheels. The
  coefficient of kinetic friction of the cars
  tires on the dry asphalt is ?k 1. How long will
  it take for this car to stop and how far it will
  move after applying the brakes?

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• Free body diagram of the car

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• Free body diagram of the car

n
D
fk
w
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In the vertical direction

• ay 0 therefore the net vertical force must be
  0
• n -w
• fk n ? K -w ? K -mg ? K
• Notice that we are only interested in the
  magnitude of normal force. The direction of the
  force of friction is always opposite to the
  velocity!

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In the horizontal direction

• The direction of the force of friction is
  opposite to the velocity!
• The friction force
• fk n ? K -w ? K -mg ? K
• The drag force is small at this speed compared to
  the braking (friction) force so we will neglect
  it for now.

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In the horizontal direction

• v0 50 km/h 50 1000/3600 13.9m/s
• We chose the coordinates for v0 to be positive
• The magnitude of the horizontal acceleration of
  the car a
• ax fk /m g ? K - 9.8m/s2 (why -)
• v2 v20 2a (x- x0)
• v 0
• (x- x0) is the braking distance. Choose
  coordinates in such a way that x0 0
• v20 - 2ax
• x -v02/2a v02/2g ?K 9.7m

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What will be the braking distance if the asphalt
is wet and ?K 0.5 not 1

• The braking distance will be twice longer
• The braking distance will remain the same
• The braking distance will be twice shorter
• The braking distance will be four times longer
• The braking distance will be four times shorter

x v02/2g ?K
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What will be the braking distance if the asphalt
is wet and ?K 0.5 not 1

• x v02/2g ?K 19.4m
• The coefficient ?K is twice smaller
• Notice that the braking distance is inversely
  proportional to ?K
• So the braking distance must be twice longer.
• x 19.4m (29.7m)

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What would be the braking distance if the initial
speed was 100km/h instead of 50km/h and ?K 0.5

• x v02/2g?K
• x 19.4 m at 50 km/h and ?K 0.5
• When the v0 doubles the distance x gets 4 times
  larger because x is proportional to v02
• x100 419.4 77.6 m

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What is the difference between a 4 wheel drive
car and a regular car?

• Drive
• Regular car 2 wheels connected to the engine
  and propelling the car
• 4 wheel drive car 4 wheels connected to the
  engine and propelling the car
• Brakes
• All cars have brakes on all 4 wheels

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Is friction good or bad?

• Walking, biking, driving
• Fabric

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Drag force in gases and liquids

• Drag force increases with velocity
• At very low speed it is proportional to velocity
• At higher speeds it is proportional to velocity
  square
• Accelerating object and drag force
• Terminal velocity

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Terminal velocity of

• Falling object
• car with engine providing a constant force
• boat with engine providing a constant thrust
  force

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Weight and apparent weight

• What is weight and how do we measure it.
• A balance scale

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Weight and apparent weight

• What is weight and how do we measure it.
• Spring scales

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Weight of a person in an elevator