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D O N OW : A ball is rolling slowly across a soccer field after being kicked. 1. Draw a force diagram. 2. Are the forces balanced or unbalanced? 3. Describe the motion of the ball and what will happen to it. Use Newton’s Laws to explain. Plea se take out your hom ewo rk and your labs! !!

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F ABULOUS F RICTION !!! 10/26/10

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F RICTION – THE TWO TYPES Static – the friction that prevents an object from moving; it determines the threshold of motion (what force it will take for a force to begin moving from rest) Kinetic (rolling) – friction that opposes motion; if unbalanced it will result in a deceleration of an object.

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W HAT DOES FRICTION DO ? Friction is the evil of all motion!!!! http://www.fearofphysics.com/Friction/ friction.html

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T HE COEFFICIENT OF FRICTION represented by the Greek letter µ Static: µ s Kinetic: µ k constant for every surface (i.e. µ is the same for ice, etc.) 0<µ<1 Units: none!

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C ALCULATING F ORCE OF F RICTION Static: F f < µ s F N Kinetic: F f = µ k F N

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F ap : the applied force; any force applied to an object by another object.

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E XAMPLE : F N =45N, µ =.11, F f ? F f =µF N Ff=(45)(.11)= 4.95N

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C ONSTANT VELOCITY When velocity is constant, there is no acceleration and all forces are BALANCED F f = F ap F N =F g FNFN

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P RACTICE : Please use the rest of class to work on “Introduction to Friction.” #4: Ff = ?

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D O N OW (11/23): A car is traveling at constant velocity. 1. The thrust f 2. a force diagram. 3. Are the forces balanced or unbalanced? 4. Describe the motion of the car and why. Use Newton’s Laws to explain. *Ple ase cons olid ate you r Do No w’s for coll ecti on and take out you r extr a cred it if you hav e it!!!

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D O N OW (11/23): A box is pushed across a table at constant velocity. 1. The applied force is 50 N and the normal force is 125 N. what is the coefficient of friction? 2. Are the forces balanced or unbalanced?

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Q UESTION : What is the acceleration due to gravity?

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Q UESTION : Using Newton’s Second Law, how do you think you could find the force of gravity? (Fg)

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F ORCE OF G RAVITY F= ma F g =mg *g= 9.8 m/s 2

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P RACTICE : Use the rest of class to work on “Thanksgiving Force Diagrams.”

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D O N OW (12/2): 1. What is the difference between static and kinetic friction? 2. Can object be in motion without being acted on by an unbalanced force? 3. What would happen if you threw an object in outer space? Use Newton’s Laws to explain.

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G ROUP P ROJECT WORK : Work with your project groups to complete either your project proposal or create a sketch, diagram, or prototype to submit at the end of class

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