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Circular Motion. CH. 9 . Rotation & Revolution. Axis A straight line through which circular motion takes place All points on object orbit around the axis All rotation/revolution requires an axis Rotation Object rotating about an internal axis

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rotation revolution
Rotation & Revolution
  • Axis
    • A straight line through which circular motion takes place
    • 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
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
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 left
    • Why is this??
so why is there no force pushing you out from the circle
So why is there no Force pushing you out from the circle??
  • A force does not cause this…… your INERTIA does!!
  • 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 happens
  • The only actual force acting on you is the

Centripetal Force

centripetal force1
Centripetal Force
  • Centripetal means “center- Seeking”
  • 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 force2
Centripetal Force
  • Inertia wants to take objects in a tangent line, to the circular path
  • Inertia is why you feel like your being pushed outward
    • 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
Centrifugal force
  • Spin cycle in laundry
  • Phone sliding off dashboard
  • Dog shake
the rotor
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.

videos
Videos
  • “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
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
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
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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