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Unformatted text preview: 1 Lecture 7 Circular Motion Physics 111, Summer 2011, June 22, Lecture 7 Summary To apply three Newtons laws of motion, we need to remember what can happened (i) if the net force is zero, or (ii) if the net force is not zero; finally that (iii) each force of action produces the force of reaction. 2 A free-body diagram (force diagram) shows all the forces acting on the object, means free of its surroundings. Kinetic friction occurs between two objects which have a contact and one slides over another : Static friction exits between two objects which have a contact and one do not slide over another: Kinetic coefficient of friction typically is less than static coefficient; hence, for the same two materials, kinetic friction is smaller than static friction. The origin of friction are microscopic irregularities of a surface. N k fr F F = N s s fr F F ) ( Mass is constant, weight depends on the acceleration due to gravity. Physics 111, Summer 2011, June 22, Lecture 7 Useful trick : setup x-axis along the ramp. Hence, the net force acting on the object will be . sin sin mg W W F x net = = = x W So, we dont need to know the mass of the object! sin g a x = 2 2 / 9 . 4 ) 30 )(sin / 8 . 9 ( s m s m a x = = 30 = y W net x F W = = y N W F mg W = y x 3 A box slides down on incline with an angle = 30 without friction. Find the acceleration of the box....
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