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PH 201

PH 201. Dr. Cecilia Vogel Lecture 23. REVIEW. equilibrium stable vs. unstable static. OUTLINE. eqlb universal gravitation. Static Equilibrium. Static means not moving equilibrium means no net force And no net torque This is the object at rest, remaining at rest.

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PH 201

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  1. PH 201 Dr. Cecilia Vogel Lecture 23

  2. REVIEW • equilibrium • stable vs. unstable • static OUTLINE • eqlb • universal gravitation

  3. Static Equilibrium • Static means not moving • equilibrium means no net force • And no net torque • This is the object at rest, remaining at rest

  4. 2-D Static Equilibrium • Generally, need to consider • 3-D forces, • and torques around two axes • We will do 2-D problems • SFx=0, SFy=0, • St=0 around axis parallel to z-axis • For 3-D problems, easiest to do 2-D twice • Side view and overhead view, for example

  5. Eqlb Examples • crane holding a block • The system in Figure 12-37 is in equilibrium. A concrete block of mass 222 kg hangs from the end of the uniform strut of mass 45.5 kg. Angle f= 30.0° and q= 45.0°.Find the tension T in the cable.Find the force on the strut from the hinge

  6. Hinges and Hips • The socket (or outer hinge part) exerts • on the ball (or inner hinge part) • a circular surface can exert a normal force

  7. Eqlb Examples • crane holding a beam, cont’d

  8. Eqlb Example Answer • 3 eqn, 3 unknown T =6553 N FHx =5675 N FHy=5898 N Hinge force vector is not along strut

  9. Gravity • Recall • Fg = mg and U = mgh • only true near earth surface! • further from the earth, the force gets weaker

  10. Universal Gravity • Newton’s Universal Law of Gravity: • m1 and m2 are the two masses • r is distance between the two masses • G is gravitation constant 6.67X10-11 Nm2/kg2 • PE always negative • farther, smaller neg, higher PE • negative F means opposite the vector r • means it is always attractive

  11. Gravity Pulls on Earth • Which pulls harder on the earth, the moon or the sun? • moon is closer • sun more massive • hmm….

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