Circular Motion

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Circular Motion
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Vocabulary for Circular Motion

• Period(T) the time it takes for one rotation or
  revolution. Measured in seconds.

• Revolution traveling about an external axis,
  kid on merry-go-round

• Centripetal force a center-seeking force, causes
  circular motion

• Rotation spinning about an internal axis, an
  ice skater spinning

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Two types of speed to think about

• Linear Speed is total distance divided by time.
  In circular motion, total distance is equal to
  circumference or 2pr. The time is equal to the
  period.

• Rotational speed (AKA angular speed) refers to
  the number of rotations/revolutions per unit of
  time. A common unit for this is RPM. Any
  guesses on what RPM stands for?

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Linear Speed

• With linear speed, a point on the outer edge of
  an object moves a greater distance in one
  rotation than a point closer to the center of the
  object.

Two ladybugs sit on a rotating disk, as shown in
the figure (the ladybugs are at rest with respect
to the surface of the disk and do not slip).
Ladybug 1 is halfway between ladybug 2 and the
axis of rotation. A) Which has a great linear
speed? B) Which has a greater rotational speed?
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Speed/Velocity in a Circle
Consider an object moving in a circle around a
specific origin. The DISTANCE the object covers
in ONE REVOLUTION is called the CIRCUMFERENCE.
The TIME that it takes to cover this distance is
called the PERIOD.
Speed is the MAGNITUDE of the velocity. And while
the speed may be constant, the VELOCITY is NOT.
Since velocity is a vector with BOTH magnitude
AND direction, we see that the direction, and
thus, the velocity, is ALWAYS changing.
We call this velocity, TANGENTIAL velocity as its
direction is drawn TANGENT to the circle.
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Centripetal force

• centripetal force- a center directed force that
  causes an object to move in a curved (sometimes
  circular) path.
• Centripetal means "center-seeking" or "toward the
  center".
• It is any force that is at a right angle to the
  path of a moving object that has a circular
  motion.
• As you can see by this picture, when on a roller
  coaster that has a loop, the passengers
  experience centripetal force, as well as
  inertia.Centripetal force can be found with the
  following equations
• Fc ma mv2/r m4p2r/T2

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Analyzing the forces

• http//library.thinkquest.org/27948/carousel.html

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Centripetal or Centrifugal Force ???
Centripetal and Centrifugal Forcescentrifugal
force- an apparent outward force on a rotating or
revolving body. It is fictitious. Due to the
fact that our velocity is constantly tangent to
the circular path we are following, a force has
to constantly be pushing/pulling us toward the
center. The pressure we feel as a result of the
inward force tricks our minds into thinking we
are being pushed outward.
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Video for review
Centrigal Force- NOT http//www.youtube.com/watch?
vSav9vQ663u4
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Centripetal Acceleration
If an object moves in a circle it is accelerating
and there has to be an unbalanced force acting on
it. A force that causes circular motion is always
toward the center. Acceleration is always in the
same direction as the force (so centripetal
acceleration is toward the center). If, for
instance, you tie a weight on a string and swing
it around over your head in a circle you can feel
the centripetal force needed to keep it going in
a circle. If you let go, that is remove the
centripetal force, it will fly off in a straight
line in whatever direction it happens to be going
at that moment. Centripetal acceleration is found
using the following equations ac v2/r
4p2r/T2
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Drawing the Directions correctly
So for an object traveling in a counter-clockwise
path. The velocity would be drawn TANGENT to the
circle and the acceleration would be drawn
TOWARDS the CENTER. To find the MAGNITUDES of
each we have
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In what direction will the car continue if the
curve he is about to travel around is solid ice
starting from point B?
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In what direction will the car continue if the
curve he is about to travel around is solid ice
starting half way between point B and C?
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A .013-kg ball is attached to a 0.93-m string.
The ball is swung in a horizontal circle, making
one revolution in 1.18 s. Find the tension force
exerted by the string on the ball.
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What is the direction of the force that acts on
clothes in the washing machine in the spin cycle?
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What is the name and direction of the force on
the ball below?What is the name and direction of
the acceleration?
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Label as Rotating(x) or Revolving(y)

• Earth traveling around the sun
• Moon traveling around the earth
• Earth spinning
• Kid on seesaw
• Person on roller coaster traveling around a loop
• Dog chasing his tail
• Ice Skater Spinning
• A door
• Discus thrower (in reference to the person)
• Discus thrower (in reference to the disk)

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Book AssignmentPage 172-173, 49-54