Physics and Driving

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Physics and Driving
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Demonstration Design
Light Sensor
Light
Truck
Zero Line
As the truck passes the light, the light sensor
turns off the power to the truck, ensuring that
the truck begins stopping at the same point on
each run.
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What do we need to know?

• Velocity of the truck on low speeddtv
• Velocity of the truck on high speeddtv

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Derivation of Stopping Distance

• Equations you are familiar with
• Fma
• WFd
• W?K
• Kmv2/2

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Derivation of Stopping Distance
Fd?K madmvf2/2 mvo2/2 We see that the
stopping distance is mass independent. advf2/2
vo2/2 In our case, vf0. The equation
becomes d -vo2/2a Can you explain the negative
sign?

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d -vo2/2a

• Remember we are working with vector values.

v
a
d
While the value of v is positive, the value of a
is negative. Negative signs will cancel to give
a positive distance.
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• d -vo2/2a cvo2 where c is a constant
• volow
• volow2
• dlow
• c
• Now predict stopping distance on high speed
• vohigh2
• dhigh

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A Note About Error

• Data varies over a range depending on the
  equipment and the person making the measurements.
  This is not uncommon.
• The more measurements used to find an average
  value, the more accurate that value will be.
• If the experimental value is equal to the
  expected value within error, the experimental
  value is correct.
• What are some sources of error in the
  demonstration?

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So what does this mean?

• When cars hit objects, the car is compressed, not
  the object. The faster the car is going, the
  more it will compress, and the greater the
  likelihood of injury.

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Review of Friction

• Sliding friction
• Acts between two surfaces
• Opposes the direction of motion.
• Is proportional to the normal force.
• Varies with the slipperiness of the surfaces.
• F µN

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Friction

• The friction between the road and a cars wheels
  is called traction.
• Traction allows cars to accelerate and to change
  direction.
• What happens when the surface the wheels contact
  changes (the coefficient of friction is changed)?

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Accelerating rolling object How does friction act?
Applied force
rotation
acceleration
velocity
Normal force
weight
friction
(a)
(b)
(c) No friction is acting
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Force diagram of a accelerating rolling object
Applied force
rotation
acceleration
velocity
Normal force
weight
friction
The answer is (b)
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Why does rolling friction act this way?

• Imagine that no friction exists between the tires
  and the road. When the wheels are rotated, they
  will simply spin in place (think of peeling out)
• In order to make the tire travel forward, the
  tire must bite into the road at the point of
  contact - i.e., friction must exist in the
  direction opposite the motion.

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• Imagine the spinning wheel again. The motion at
  the point of contact is towards the left.
• The friction must act to the right in order to
  make the wheel roll!
• Hence, the road pushes the tire.

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Decelerating rolling object How does friction act
in?
Applied force
rotation
velocity
acceleration
Normal force
weight
friction
(a)
(b)
(c) There is no friction force acting
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Force diagram of a decelerating rolling object
Applied force
rotation
velocity
acceleration
Normal force
weight
friction
The answer is (a)
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Why does friction act this way?

• Imagine a tire rolling along. Now imagine that
  the rotation of this tire is slowed by some
  internal force (e.g. brakes).
• Remember Newtons first law an object in motion
  will stay in motion unless an external force acts
  upon it.
• The tires inertia will carry it on at the same
  velocity (though slipping) unless rolling
  friction slows the speed of the tire.
• This is what happens when your car skids.

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Friction

• Applied force speeds up or slows down rotation
  friction force is what actually accelerates or
  decelerates the object.
• Friction force varies with amount of applied
  force.
• The maximum friction force is determined by the
  coefficient of friction between the wheel and
  surface. If the applied force exceeds the
  maximum friction force, slipping occurs.

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What force is stopping the truck?

• FRICTION
• Internal friction acting on the wheel axis is
  slowing the rotation of the wheels.
• However, according to Newtons first law, an
  external force must act upon an object to change
  its inertia.
• Rolling friction is what slows the moving body.

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ABS

• Anti-lock breaks work by keeping the breaking
  force on the tires equal to or less than the
  friction force.
• The rotation then slows at the same rate the car
  decelerates, so the car does not skid.