CH. 9 Circular Motion

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 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 left

Why is this??

?

𝐹 𝑐=𝑚𝑣𝑡❑

2

𝑟

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 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 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 Spin cycle in laundry Phone sliding off dashboard Dog shake

The RotorPeople 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

“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 eachFull 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