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Download Physics 130 - UND: University of North Dakota
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10/9 About “Work” Text: Chapter 6 Energy HW 10/9 “Skidding Objects” due Friday 10/11 Exam 2 Thursday, 10/17 5-7 Wit 116 6-8 Wit 114 (only if needed) Please send email if other time needed Work, Fother x Other means not Weight How should we multiply vectors? Moving in a circle at constant speed as in lab. 0 + Work = 0 PEg,i KEi PEg,f KEf Since Ei = Ef, no Work is done on the ball. But... There is a net Force that points toward the center of the circle! We are OK if we note that the Force is perpendicular to the velocity and x. Multiplying Vectors x = 4m F = 6N +24J Force in the direction of motion -24J 0J Force opposite the direction of motion Force perpendicular to the direction of motion -24J Force opposite the direction of motion 0J Force perpendicular to the direction of motion ?J Hmm…... How much Work is done? F = 1000 N 37o Frictionless Table How much Work is done? Vertical component does no Work, horizontal component does. Work = F x cos is the angle between F and x Work = 1000(5)cos37° 4000 J F = 1000 N 37o x = 5m Frictionless Table F = 1000 N 37o How much Work is done? What is the block’s velocity if it starts from rest? m = 10 kg 0J + 4000 J = 4000 J T = 1000 N T = 1000 N 37o 37o x = 5m Kef = 1/2 mvf2 = 4000 vf = 28 m/s How much Work is done? A block slides down a frictionless ramp. Since, at any instant, the Normal Force is perpendicular to the velocity and x, no Work is done at all! You can find velocities anywhere if you know the altitude difference.
