CH. 9

Circular Motion
CH. 9
Rotation & Revolution
 Axis
 A straight line through which circular motion takes
 All points on object orbit around the axis
 All rotation/revolution requires an axis
 Rotation
 Object rotating about an internal axis
 Ex. Daily motion of the Earth, spiral football
 Revolution
 Object rotating about an external axis
 Ex. Yearly motion of the earth
Centripetal Acceleration
 Can an object being moving at a constant speed but
also have a changing velocity?
 Yes, can be changing direction
 As is the case when an object is moving w/ a constant
speed in a circular path
 Changing direction  changing velocity acceleration
  =

Centripetal Force
 When driving in a circle, in what direction is a force
acting on you?
 Pushing you outward from the circle, or inward?
 If you are swinging a yo-yo in a circle, and the string breaks….
What path does the yo – yo take??
 Ans. -- Inwards, toward the center of the circle
 Ans -- yo- yo goes in a path tangent to the circle
 HOWEVER, People commonly think there is a force
pushing you out from the circle
 Feels like you are being pushed outward
 Example ….. The Rotor- amusement park ride, a centrifuge,
CD on your dashboard moving to the right when your turning
 Why is this??


So why is there no Force pushing
you out from the circle??
 A force does not cause this…… your INERTIA
 Inertia makes you want to stay in a straight line,
and by going in a circle, you are fighting your
own inertia
 This is how Rotor works, and why CD on dashboard
 The only actual force acting on you is the
Centripetal Force
Centripetal Force
 Centripetal means “centerSeeking”
 Force pushes you toward
the center of the circle
 Is the force that keeps you
moving in a circle, and
keeps your inertia from
taking you in a straight line
Centripetal Force is affected
by.. Mass (m),
linear speed (vt),
and radius (r)
Centripetal Force
 Inertia wants to take objects in a tangent line, to
the circular path
 Inertia is why you feel like your being pushed
 This outward pushing is sometimes called the
Centrifugal Force
 but it is not actually a force, is only inertia
 Every object that moves in circular motion must
experience a centripetal force from somewhere
Centrifugal force
 Spin cycle in laundry
 Phone sliding off dashboard
 Dog shake
The Rotor
People Stand with backs
against wall of a large
cylinder, cylinder then
starts spinning, and people
are seemingly pushed
against the wall, then floor
drops, and people are stuck
against the wall.
 “G-Forces”
 NASA Centrifuge
 Centrifuge Training
 9G test run
 Gross (negative G’s)
 Another 9 G test run
How do we describe how fast
something is rotating??
 Speeds for objects in a straight line are called linear
(or tangential) speeds,
 Linear speeds are a rate at which an object covers a
certain distance (v =d/t)
 Ex. Unit – m/s , km/hr , mph
 Can’t express speeds of rotation with a linear speed,
 b/c objects at different points on the rotating object have
different linear speeds
 Rotational speed
 Expresses the rate at which an object rotates through a
portion of a circle ( an angle)
 Ex. Unit --- RPM’s
Below, a record spinning on a axis
through its center (black dot)
 Faster linear speed, Star or Smiley??
Smiley, travels a greater distance for each
Full spin.
 Faster rotational speed, Star or smiley??
 Both the same, b/c entire record is rotating at the same rate
Are all people on Earth moving at
the same speed??
 Earth is rotating about an axis through its poles
 So that means we are all moving since we are all on
the Earth.
 Are some of us moving with a greater LINEAR
SPEED than others??
 Yes, closer to the Equator, the faster you are moving….
Closer to poles, the slower you are moving
 Are some of us moving with a greater ROTATIONAL
SPEED than others??
 No, all people on earth have same rotational speed,
because Earth is spinning at the same rate everywhere

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