CH. 9

1. Circular Motion CH. 9

2. 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

3. 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

4. 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??

5. 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

6. 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)

7. 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

8. Centrifugal force • Spin cycle in laundry • Phone sliding off dashboard • Dog shake

9. 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.

10. Videos • “G-Forces” • NASA Centrifuge • Centrifuge Training • 9G test run • Gross (negative G’s) • Another 9 G test run

11. 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

12. 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

13. 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