Lecture 8 PowerPoint - Today Friction Calming Words about...

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1 Today Friction Calming Words about the Midterm Q & A 1 Wednesday, 3 October, 12
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2 Kinetic Friction Moving objects : kinetic friction force (constant!) μ k : coe ffi cient of kinetic friction n: normal force. Direction is parallel to the sliding surface (perpendicular to the normal force) and opposite the (instantaneous) velocity. v 2 Wednesday, 3 October, 12
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3 Typical Problems A book is sliding on a table with an initial speed of 2.5 m/s. Due to friction, the book comes to a stop after 0.5 m. If the book’s mass is 800 g, what is the coe ffi cient of kinetic friction? 3 Wednesday, 3 October, 12
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3 Typical Problems A book is sliding on a table with an initial speed of 2.5 m/s. Due to friction, the book comes to a stop after 0.5 m. If the book’s mass is 800 g, what is the coe ffi cient of kinetic friction? A 1.6 kg book (same surface) is sliding with the same initial speed on the same table. What is the sliding distance now? A. 0.125 m B. 0.25 m C. 0.50 m 3 Wednesday, 3 October, 12
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Solution Multiple choice part: Acceleration due to friction does not depend On the mass. The friction force itself does. Due to part 1, same acceleration leads to same stopping distance. 4 Wednesday, 3 October, 12
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5 5 Wednesday, 3 October, 12
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6 Friction You are trying to move a heavy file cabinet. You increase your pushing force until the cabinet finally starts to move. If you keep pushing with the force that was required to set the cabinet in motion, the file cabinet will A. travel at constant speed. B. slow down. C. accelerate. PhET : Friction simulation/forces-and- motion 6 Wednesday, 3 October, 12
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6 Friction You are trying to move a heavy file cabinet. You increase your pushing force until the cabinet finally starts to move. If you keep pushing with the force that was required to set the cabinet in motion, the file cabinet will A. travel at constant speed. B. slow down. C. accelerate. F fr F a f s = F a static kinetic PhET : Friction simulation/forces-and- motion 6 Wednesday, 3 October, 12
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7 Friction You are trying to move a heavy file cabinet. You increase your pushing force until the cabinet finally starts to move. If you keep pushing with the force that was required to set the cabinet in motion, the file cabinet will A. travel at constant speed.
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