Statics

1
Statics
2
Static Equilibrium

• There are three conditions for static
  equilibrium.
• The object is at rest
• There is no net force
• There is no net torque

3
Static Forces

• With no motion, forces must sum to zero.
• Each component is treated separately

• Find the tension in the ropes.
• T is the tension to the left
• S is the tension to the right
• W is the weight downward
• Vertical equality
• T sinq W mg
• T mg / sinq 3.9 x 103 N
• Horizontal equality
• S T cosq 3.4 x 103 N

q 30º
m 200 kg
4
Offset Forces

• Balanced forces do not insure static equilibrium.
• The center of mass is stationary, but the object
  rotates.

F
-F
5
Static Torques

• With no rotation, torques must sum to zero.

• A ladder leans against a wall with friction
  against the floor.
• Length l
• Angle q
• Coefficient of static friction m
• The sum of torques gives

FNW
FNF mg
Ffr ? mmg
-mg
q
6
Pivot Point

• The calculation of torques is independent of the
  pivot point.

• Select the wall for the pivot.

FNW
FNF mg
Ffr ? mmg
-mg
q
7
The Lever

• The lever is a simple machine that increases
  force.
• The downward force is multiplied by the lever arm
  of the stick to become a torque.
• The torque on the rock matches the torque from
  the person.

8
Human Lever

• The elbow joint is a pivot point for a lever.
• Assume all forces are vertical.
• Levers may not reduce force.

9
Stability

• When is an object in static equilibrium?
• Check the forces and torques
• If the torques are from gravity, find the center
  of mass

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