PH 201

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
• SFx0, SFy0,
• St0 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

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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, contd

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Eqlb Example Answer

• 3 eqn, 3 unknown

T 6553 N
FHx 5675 N
FHy5898 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

• Newtons 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.