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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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Indicate what effect the change will have on the acceleration.

Indicate what effect the change will have on the acceleration.

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