AP Physics 1 Crash Course for Spring 2021 ๐ŸŽก

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3.7 Free-Body Diagrams for Objects in Uniform Circular Motion

#uniformcircularmotion

#forceofgravity

#force

#projectilemotion

โฑ๏ธย ย 2 min read

written by

Peter Apps

peter apps

June 8, 2020


Essential Knowledge 3.B.1 ๐Ÿ“

If an object of interest interacts with several other objects, the net force is the vector sum of the individual forces. Projectile motion and circular motion are both included in AP Physics 1.

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Essential Knowledge 3.B.2 ๐Ÿ–Š

Free-body diagrams are useful tools for visualizing forces being exerted on a single object and writing the equations that represent a physical situation.

Free-body or force diagrams may be depicted in one of two waysโ€”one in which the forces exerted on an object are represented as arrows pointing outward from a dot, and the other in which the forces are drawn at the point of exertion.

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Imagine the colored rectangles above depict a roller coaster at different points during a loop, red being the top, orange the right, green the left, and yellow the bottom. Letโ€™s practice with free-body diagrams for uniform circular motion by drawing one for each position of the roller coaster.

Remember, in this instance, there is only centripetal acceleration, no tangential acceleration. If there is a centripetal force causing centripetal acceleration, it must point to the center. The normal force is the only force in this case that points to the center. Normal force doesn't always point to the center; there are always exceptions! Solve each problem case by case - use your free-body diagrams!

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The free body diagram above depicts the roller coaster at the top of the loop, where Normal Force and Force of Gravity are both pointed downwards. This gives us a net force equation of Fnet = Fn + Fg.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2FScreen%20Shot%202020-04-13%20at%2012.16.56%20PM.png?alt=media&token=de789e71-dbff-4195-9ac2-2e3931ddcc59

The free body diagram above depicts the roller coaster at the bottom of the loop, where Normal Force is pointed upwards and Force of Gravity is pointed downwards. This gives us a net force equation of Fnet = Fn - Fg.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2FScreen%20Shot%202020-04-13%20at%2012.17.34%20PM.png?alt=media&token=ecb2d2a2-54a0-430e-85a7-b333ed70b958

The free body diagram above depicts the roller coaster at the left of the loop, where Normal Force is pointed rightwards and Force of Gravity is pointed downwards. This gives us a net force equation of Fnet = Fn.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2FScreen%20Shot%202020-04-13%20at%2012.19.31%20PM.png?alt=media&token=fe1e95b1-8c6b-4f32-93be-ae9e7bfb80af

The free body diagram depicts the roller coaster at the right of the loop, where Normal Force is pointed leftwards and Force of Gravity is pointed downwards. This gives us a net force equation of Fnet = Fn.

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