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Newton’s Laws June 5, 2012

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Sample Problem 1 A tetherball leans against a smooth, frictionless post. The string is 1.4 m long and the ball has a radius of m with a mass of kg. What is the tension in the rope? What force does the pole exert on the ball?

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Sample Problem 2 Two blocks are held in place on a frictionless incline. The mB = 10 kg and mA = 12 kg. The incline is tilted at an angle of 37o to the horizontal. Determine the tension in each rope.

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Friction Friction is a resistive force. Kinetic friction is resistive to motion and static friction resists motion in the first place. In general, static friction is greater than kinetic friction

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Kinetic Friction Will always point in a direction opposite to the direction of motion Ff = µkFn

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Sample Problem 3 A box of mass 25 kg is pushed across a floor at a constant speed of 2 m/sec. What force is needed to push the box if there is a coefficient of friction of 0.2 between the box and the floor?

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Sample Problem 4 The weight of the crate is 500 N and there is a coefficient of friction between the crate and the floor of If the worker pulls with a 400 N force, what is the acceleration of the crate?

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Sample Problem 5 A pig slides down a chute inclined at 30o. If there is a coefficient of friction between pig and slide of 0.6, what is the acceleration of the pig down the slide?

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**Static Friction Resists the desire of an object to move.**

Range of possibilities though we will typically look at the maximum case.

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Static Friction

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**Static Friction Harder to predict which way it will go. Ffs < µsFn**

Typical cases are the limits where Ffs = µsFn

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Sample Problem 6 A person on a stationary 125 N box with a force of 75 N at 30o below the horizontal. Determine the coefficient of static friction if the box just starts to move.

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Sample Problem 7 If the coefficient of static friction between the box and the truck bed is 0.6, what is the greatest acceleration the truck can have without the box slipping?

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Newton’s 3rd law and Friction Lecture 4. Newtons First Law If no net external force is applied to an object, its velocity will remain constant ("inert").

Newton’s 3rd law and Friction Lecture 4. Newtons First Law If no net external force is applied to an object, its velocity will remain constant ("inert").

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