# lec13 - Lecture 13 P112 Agenda for today Drag Dynamics...

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Lecture 13 P112 Feb 18, 2008

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Agenda for today Drag Dynamics
Graph of force due to friction versus applied force F A F A F K F N f F

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Fluid Resistance (Drag)
Fluid resistance Direction? Opposes relative motion of fluid and object. Magnitude? Will depend on the velocity, shape of the body, properties of the fluid Observation: at low speed it is proportional to v, at high speed, it is proportional to v 2

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Drag forces Oppose motion of an object through fluid (liquid or gas) 2 approximate force laws: 1. Linear drag (low speeds, viscous fluids) 2. Quadratic drag (high speeds, low viscosity fluids (gases) f d v (rel. to fluid) f d = bv f d = Dv 2
For fast things moving through air, use the quadratic drag formula Airplanes, raindrops, cars at high speed..

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Quadratic Drag force “Drag coefficient” c d Object’s front cross-sectional area A Fluid’s density ρ D = 1 2 c d A r f d = Dv 2
Air Drag in Auto Design: D = (1/2) C ρ A v 2 C: C:

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Air Drag in Auto Design: D = (1/2) C ρ A v 2
Air Drag in Auto Design: D = (1/2) C ρ A v 2 Jeep Cherokee: C D = 0.51, MPG=21

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Air Drag in Ski Jumping: D = (1/2) C ρ A v 2
Air Drag in Luge: D = (1/2) C ρ A v 2 Record luge speed: 85 mi/h

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Air Drag in Cycling: D = (1/2) C ρ A v 2
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