Concept Development 9-1

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Concept Development 9-1

• Created for CVCA Physics
• By
• Dick Heckathorn
• 4 January 2K 5

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Circular Motion

• a. If there is no relative motion between the
  train and the edge of the platform, how fast must
  the train move compared to the rim speed of the
  rotating platform?

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Circular Motion

• b. Why is the stairway located at the center of
  the platform?

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Circular Motion

• a. How is the size of the round platform and
  train speed related to the amount of time that
  passengers have for boarding?

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Circular Motion

• b. Why would this rotating platform be
  impractical for high speed trains?

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Circular Motion

• a. The platform is not rotating on its axis and
  the people are at rest.

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Circular Motion
b. Make a sketch of the missing people to show
how they must lean in comparison.

• b. When the platform rotates, the person in the
  middle stands as before. The person at the edge
  must lean inward as shown.

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Circular Motion

• The sketch on the left shows some toy ducks
  floating on the water.

The sketch on the right shows the container
rotating about a central axis at constant speed.
Make a sketch showing the orientation of the
other two ducks with respect to the water surface.
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Circular Motion
The sketch on the right show the water surface
when the tire and water rotate about its central
axis.

• The sketch on the left shows the water surface
  when the tire is not rotating.

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Circular Motion
Draw the shape of the water surface in the
cross-sectional view above.

• Now suppose the tire is rotating about the same
  axis while orbiting in outer space.

Scale up the rotating tire model to a rotating
space habitat orbiting in space.
Where would the people be and how would they be
affected?
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Concept Development 9-2

• Created for CVCA Physics
• By
• Dick Heckathorn
• 15 December 2K3

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Accelerating Circular Motion
v
a
a. Your body lurches backward

• Youre in a car at a traffic light. The light
  turns green and the driver steps on the gas.

b. The car accelerates forward
c. Force on car acts forward
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Accelerating Circular Motion
v
a
a. Your body lurches forward

• Youre driving along and approach a stop sign.
  The driver steps on the brakes.

b. The car accelerates backward
c. Force on car acts backward
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Accelerating Circular Motion
a
v
a. Your body leans outward

• You continue driving and round a sharp curve to
  the left at constant speed.

b. Direction of acc. is inward
c. Force on car acts inward
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Accelerating Circular Motion
4. In general, the directions of lurch and
acceleration, and therefore the direction of
lurch and force are
the same not related opposite
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Accelerating Circular Motion
a. If it moves faster, its direction changes
faster slower
5. The whirling stones direction of motion keeps
changing.
b. This indicates that as the speed increases,
acceleration increases decreases stays the
same
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Accelerating Circular Motion
6. Consider whirling the stone on a shorter
string that is, of smaller radius
a. For a given speed, the rate that the stone
changes direction is less more
the same
b. This indicates that as the radius decreases,
acceleration increases decreases stays the same
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Centripetal Force p 40
Ftable on rock
Fstring on rock
Fearth on rock
1. A rock tied to a post moves in a circle at
constant speed on a frictionless horizontal
surface. All the forces acting on the rock are
shown.
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Centripetal Force p 40
Ftable on rock
Fstring on rock
Fearth on rock
a. The vector responsible for circular motion is
Fstring on rock
b. The net force on the rock is Fstring on rock
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Centripetal Force p 40
Fequilibrium
T
Fnet
W
2. The rock is tied to a string and swings in a
circular path. It is not resting on a surface. No
friction. Find the net force.
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Centripetal Force p 40
T
Fnet
W
a. What is the direction of Fnet? to center
b. Does Fnet lie in plane of circular path? Y
c. Is Fnet horizontal Component of T? Yes
d. Is Fnet the centripetal force? Yes
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Centripetal Force p 40
Fdisk on rock
Ffriction
Fearth on rock
3. The rock rides on a horizontal disk that
rotates at constant speed about the vertical
axis. Friction prevents the rock from sliding.
Draw and label all vectors.
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Centripetal Force p 40
Fdisk on rock
Ffriction
Fearth on rock
b. Which force is centripetal? Ffriction
c. Which force provides net force? Ffriction
d. Why do we not say net force is zero?
because centripetal acceleration is not zero
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Centripetal Force p 40
Ffriction
Fwall on rock
Fearth on rock
4. Now the rock is held in place by friction
against the inside wall of the rotating drum.
Draw and label all forces that act on the rock.
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Centripetal Force p 40
Ffriction
Fwall on rock
Fearth on rock
a. Which force is centripetal? Fwall on rock
b. Which force provides net force?
Fwall on
rock
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Centripetal Force p 40
Fe
Fwall on rock
Fnet
Fearth on rock
5. The rock rests against a frictionless wall of
the cone which rotates about its vertical axis.
The rock does not slide up or down. Draw and
label all forces on the rock.
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Centripetal Force p 40
Fwall on rock
Fnet
Fearth on rock
a. Should Fnet lie in the plane of the circular
path? Yes
b. Why
It provides the centripetal force for circular
motion
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Concept Development 9-3

• Created for CVCA Physics
• By
• Dick Heckathorn
• 15 December 2K3

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• 1. From Bobs point of view, he is at rest and
  sees Suzie moving
• clockwise

counter clockwise
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• 2. When Bob rides in the opposite direction as
  the habitat rotates, Suzie sees him moving
• faster

slower
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• 3. As Bobs bicycle speedometer reading
  increases, his rotational speed

decreases
and the normal force that feels the weight
decreases.
So friction between the tires and the floor
decreases.
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• 4. When Bob gets his speed up to 30 km/hr, as
  read on his bicycle speedometer, Suzie sees him

motionless
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• 5. Bouncing off the floor a bit while riding at
  30 km/hr, and neglecting wind effect, Bob

hovers in midspace as the floor whizzes by him at
30 km/hr.
and he finds himself
in the same frame of reference as Suzie
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• 6. Bob now rides in a clockwise direction, with
  the rotation of the habitat. Now Suzie sees him
  moving

faster.
7. As Bob gains speed, the normal support force
that feels like weight
increases.
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• 8. When Bobs speedometer reading gets up to 30
  km/hr, Suzie sees him moving

60 km/hr
and Bob finds himself
pressed harder against the floor.
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• Next, Bob goes bowling. You decide whether the
  game depends on which direction the ball is
  rolled!

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Thats all folks!