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PH 201 Dr. Cecilia Vogel Lecture 23

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REVIEW equilibrium stable vs. unstable static OUTLINE eqlb universal gravitation

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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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2-D Static Equilibrium Generally, need to consider 3-D forces, and torques around two axes We will do 2-D problems F x =0, F y =0, =0 around axis parallel to z-axis For 3-D problems, easiest to do 2-D twice Side view and overhead view, for example

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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 = 30.0° and = 45.0°. Find the tension T in the cable. Find the force on the strut from the hinge

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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

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Eqlb Examples crane holding a beam, cont’d

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

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Gravity Recall F g = mg and U = mgh only true near earth surface! further from the earth, the force gets weaker

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Universal Gravity Newton’s Universal Law of Gravity: m 1 and m 2 are the two masses r is distance between the two masses G is gravitation constant 6.67X10 -11 Nm 2 /kg 2 PE always negative farther, smaller neg, higher PE negative F means opposite the vector r means it is always attractive

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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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