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Forces and Equilibrium

Forces and Equilibrium. Fluid Resistance or Drag Force. Fluid resistance or air resistance is a force in the opposite direction of the object’s motion through that fluid At low speeds this force is proportional to the speed of the fluid. The units for k are Ns/m. Air Resistance.

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Forces and Equilibrium

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  1. Forces and Equilibrium

  2. Fluid Resistance or Drag Force • Fluid resistance or air resistance is a force in the opposite direction of the object’s motion through that fluid • At low speeds this force is proportional to the speed of the fluid. • The units for k are Ns/m

  3. Air Resistance • In air the fluid resistance is proportional to the velocity squared • The units for C are Ns2/m2

  4. Net Force and Fluid Resistance Terminal Velocity

  5. Net Force and Air Resistance

  6. High Speed Drag • D = drag force • C = drag coefficient (0.4 to 1.0) • ρ = density of fluid (air) • A = Effective cross-sectional area of the body

  7. Problem 1 • For a human body falling through the air in a spread-eagle position, the numerical value of C is about 0.25 kg/m. For an 80 kg sky-diver what is the terminal velocity?

  8. Answer • 56 m/s or 125 km/hr

  9. Problem 2 • A pitcher hurls a 0.145 kg baseball past a batter at 40.2 m/s (90mph). Find the resistive force acting on the ball at this time.

  10. Answer From Table

  11. Three Types of Equilibrium • Stable Equilibrium—Center of Gravity is directly below the support • Example: punching dummies • Unstable Equilibrium—Center of Gravity is above the support • Example: pole vaulter • Neutral Equilibrium—Center of Gravity coincides with the point of support, the body remains in whatever position it is placed • Example: tire on an axis

  12. Problem 1 • A man who weighs 800 N climbs to the top of a 6.0 m ladder that is leaning against a smooth wall at an angle of 60° with the horizontal. The non-uniform ladder weighs 400 N and its center of gravity is 2.0 m from the bottom end of the ladder. What must be the coefficient of friction at the ground if the ladder is not to slip?

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