AP_Physics_B_-_Newton_s_Laws_Cont_

# AP_Physics_B_-_Newton_s_Laws_Cont_ - NewtonsLawscontinued...

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Newton’s Laws - continued Friction, Inclined Planes, N.T.L., Law of Gravitation

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TWO types of Friction Static – Friction that keeps an object at rest and prevents it from moving Kinetic – Friction that acts during motion
Force of Friction The Force of Friction is directly related to the Force Normal. Mostly due to the fact that BOTH are surface forces N k kf N s sf N f F F F F F F μ α = = = = friction of t coefficien ality proportion of constant Note: Friction ONLY depends on the MATERIALS sliding against each other, NOT on surface area. The coefficient of friction is a unitless constant that is specific to the material type and usually less than one.

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Example A 1500 N crate is being pushed across a level floor at a constant speed by a force F of 600 N at an angle of 20° below the horizontal as shown in the figure. a) What is the coefficient of kinetic friction between the crate and the floor? mg F N F a 20 F f F ay F ax 331 . 0 21 . 1705 82 . 563 21 . 1705 1500 ) 20 (sin 600 1500 sin 82 . 563 ) 20 (cos 600 cos = = = + = + = + = = = = = = k k N a ay N a ax f N k f N F F mg F F N F F F F F μ θ
Example If the 600 N force is instead pulling the block at an angle of 20° above the

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## AP_Physics_B_-_Newton_s_Laws_Cont_ - NewtonsLawscontinued...

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