PHY2048_09-23-09 - Chapter 6: Force & Motion II Lecture...

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Chapter 6: Lecture 13 9/23/09
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The Centripetal Force Goals for this Lecture: Describe the drag force between two objects. Relate the rag force to the terminal speed of falling objects Brie±y review uniform circular motion Relate the centripetal force to uniform circular motion
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Drag force is the non-solid equivalent of friction When do we deal with the drag force: Whenever an object moves in a Fuid. A Fuid can be a liquid (water, molasses, . ..) or a gas (air, . ..) “Viscous” Fuid = more drag force Application point: At the surface of the moving objects Direction: Opposite to the direction of motion The Drag ±orce: D
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Magnitude: C: Drag coefFcient (Depends on the shape on an object) A: The effective cross section of the object (Depends on the shape on an object) ρ : The density of the ±uid v: The velocity of the object through the ±uid Note: If v = 0, D = 0 To keep an object moving at a speed v requires a force ² that is to v 2 (since ² net = ² - D = 0) The Drag ²orce: D D = ½ C ρ Av 2
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Example Auto fuel consumption vs velocity The frictional forces on a moving automobile consist of two major components a) a constant rolling resistance term and b) a velocity
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PHY2048_09-23-09 - Chapter 6: Force & Motion II Lecture...

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