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Rotational Motion

Rotational Motion. “Round and round. What comes around goes around, I’ll tell you why…” - Ratt. Rotation Versus Revolution. Think about things that move in a circle: Music records (for the elderly among you) Compact discs Brian Boitano spinning on ice skates in his very manly way

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Rotational Motion

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  1. Rotational Motion “Round and round. What comes around goes around, I’ll tell you why…” -Ratt

  2. Rotation Versus Revolution • Think about things that move in a circle: • Music records (for the elderly among you) • Compact discs • Brian Boitano spinning on ice skates in his very manly way • The earth in orbit around the sun • Lazy susans • The tires on your car • Cars on a circular Nascar track

  3. Rotation Versus Revolution • The differences among these is that some • rotate about an axis inside the object and • others revolve around an axis external to themselves • Which in the previous list did which one?

  4. Linear vs. Rotational Speed • So far in class we have only talked about linear speed (distance travelled in a straight line in some length of time) • Now, we introduce ROTATIONAL SPEED: • How many rotations an object makes in a certain length of time • Consider a circular rotating object. Is there any point on the disk that rotates any faster or slower than any other point?

  5. Rotational Speed • The speed of a point moving in a circle is proportional to • the distance from the center X the rotational speed • Imagine two ladybugs on a turntable. One is halfway out from the center and the other is at the edge. Which one is moving faster? How do you explain that given that the rotational speed is the same?

  6. Centripetal force • “Centripetal force” means a force that is directed in towards the center of a circle • Centripetal forces give rise to something moving in a circle • The force always points towards the center of the circle, so the direction of the force is always changing

  7. Examples of Centripetal Force • A kid whirling a rock tied to a string overhead • What supplies the centripetal force? • The earth in a (nearly) circular orbit around the sun • What supplies the centripetal force? • The clothes in a washing machine while it spins • What supplies the centripetal force?

  8. Centripetal force video • This man is more daring in his class than i am in mine…

  9. Centripetal Vs. Centrifugal • Okay, folks, these terms are often confused. • For our purposes, think of the difference as this: • Remember newton’s 3rd law? Someone say it out loud… • Well, the centripetal force pulls in towards the center. • Centrifugal force is the reaction force to the centripetal force (I.e. it pushes away from the center, not towards the center)

  10. Review questions • 1-6, 8-13, 19-24

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