Circular motion
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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?? dot)

  • 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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