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Physic 121 notes

# Physic 121 notes - Solomons Study Notes College Physics 1...

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S olomon’s Study Notes College Physics 1 Mechanics & Heat Fall 2011 Solomon Weiskop PhD [ Circular Motion & Gravity ] These Notes cover circular motion (mainly uniform but also nonuniform) and Newton’s universal law of gravitation.

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Study Notes are available to print out by registering at www.solomonlinetutor.com Solomon Weiskop PhD Solomon’s Tutoring © Copyright 2011
1 1. Uniform Circular Motion We are now going to study circular motion (i.e. objects moving in circular paths). We’ll begin with (and focus mainly on) Uniform Circular Motion. The “uniform” means that the object is maintaining a constant speed as it moves around the circle: (useful result for Uniform Circular Motion) is the circumference of a circle (of radius r) [i.e. the distance covered by the object in completing one full revolution around the circle] is the period [the time it takes to complete one full revolution around the circle] Consider a hockey puck moving at the end of a string in a horizontal circle on a tabletop. Suppose it is moving at constant speed (so that we have uniform circular motion): Despite being constant, there is nevertheless an acceleration called centripetal acceleration .

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2 Acceleration is the rate of change of velocity. Recall that velocity is a vector, having magnitude and direction. If anything about is changing, there is an acceleration. In uniform circular motion, the magnitude of the velocity vector (i.e. the speed) is constant. However, in any circular motion whatsoever the direction of is always changing (the object must stay in the circle). The acceleration associated with the changing of the direction of in circular motion is called: centripetal acceleration “centripetal” means “center - seeking” i.e. is directed towards the center of the circle (as indicated in the pictures on previous page) There is also nonuniform circular motion (where the speed is not constant). I’ll postpone a more detailed discussion of nonuniform circular motion until later (Section 6). B ut one important point I’d like to make right now: As I’ve already said, for any circular motion whatsoever (uniform or nonuniform) the direction of the velocity must necessarily always be changing. That means that for any circular motion whatsoever (uniform or nonuniform) there will be centripetal acceleration.
3 Key Point: When doing a circular motion problem, you are basically doing a force problem So you follow the General Strategy for solving force problems: You need to make a FBD showing all forces acting on the object (and also showing the acceleration). A FBD also needs axes: For circular motion the “centripetal” (i.e. center -seeking) direction is important . So I will distinguish it in my FBD’s by always choosing this direction as one of my axes: the c-axis ( c for centripetal).

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Physic 121 notes - Solomons Study Notes College Physics 1...

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