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Rotational Quantities Going in Circles Rotation vs Revolution Rotation is to spin on an axis, like a wheel; Revolution is to travel in a circle around a center point, like cars on a race track. Forces which cause motion in a circle (revolutions) If an object is moving in a circle there is a centripetal force causing the curving. Centripetal means “center-seeking” There are no centrifugal forces! Gravity In space, gravity is the force that causes the Moon to orbit (revolve around) the Earth, and the Earth to orbit the Sun. That makes it a centripetal force! Newton was the Physicist who first described mathematically gravity. Newton’s Law of Universal Gravitation Every object in the universe attracts every other object with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers. Formula to follow: An Inverse Square Law! m1m2 F G 2 r But Why? Newton didn’t say why, he just derived the math. In the centuries to follow, many laws were shown to be inverse square proportions. The reason has to do with a “flat” universe and Euclidean geometry. Einstein changed all this, but we’ll stick with Newton. Forces that cause an object to rotate Torques cause objects to spin on their axis. A torque is a force acting over a length. The length is called a lever arm. F1L1 = F2L2, as in a seesaw or mobile. Steering wheels, crowbars, and doorknob placement are all uses of torque. The center of gravity is where all the weight of an object appears to be. Rotational Inertia Objects rotating tend to keep rotating. As linear inertia is related to mass, rotational inertia is related to moment of inertia. The moment of inertia (nothing to do with time) is how an object’s mass is distributed around an axis. Keep on Rotating! The conservation of rotation, so to speak, is called the conservation of angular momentum. When an ice-skater spins slowly but then pulls her arms in, she speeds up because of conservation of angular momentum. Precession Combining angular momentum and torque (in very mathematical ways) produces a precession. The slow turn around the vertical axis of the suspended bicycle wheel. As the World Turns (ahem), the torque produced by the Sun and Moon on our planet causes the Earth to precess every 25,000 years.
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