2.2 
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1 - Science of Physics
2 - Forces and Motion      Chapter 2 at a glance      2.1 - Motion      2.2 - Force, momentum, and Newton's laws      2.3 - Circular motion      2.4 - Solving harder physics problems      2.5 - Chapter review3 - Energy Transformations4 - Waves and Sound
5 - Electricity and Magnetism
6 - Light and Optics
7 - The Atom
8 - Linear Motion
9 - Vectors and Forces
10 - Forces
11 - Newton's Laws and Gravitation
12 - Circular Motion and Gravitation
13 - Torque and Static Equilibrium
14 - Momentum and Collisions
15 - Angular momentum
16 - Power and Machines
17 - Harmonic Motion
18 - Waves
19 - Sound
20 - Electricity
21 - Electric and Magnetic Fields
22 - Electromagnetism and Induction
23 - Light and Color
24 - Geometrical Optics
25 - Electromagnetic Radiation
26 - The Properties of Matter
27 - Thermodynamics and Heat Transfer
28 - The Atom
29 - The Quantum Theory
30 - TBD
31 - TBD
32 - TBD
33 - TBD
34 - TBD
35 - TBD
36 - TBD
37 - The atomic nucleus and radioactivity
38 - Nuclear reactions
39 - Relativity
40 - Particle physics and the standard model
41 - Electronics (semiconductors)
42 - Nuclear technology
43 - Lasers and photonics
44 - Metals, alloys and materials science
45 - Communications technology
46 - Buildings and structures
47 - Energy technology
48 - Biophysics
49 - Matter and Energy, Space and Time
50 - Energy Transformations
51 - Nanotechnology
52 - Website Videos
53 - ExtraStuff
55 - Modeling Motion, Friction, and Center of Mass
56 - Work and the Conservation of Energy
Equilibrium and statics
What is equilibrium? In a static problem nothing moves, either in linear motion or rotation. Therefore, the net force must be zero and also the net torque must be zero. This condition is called equilibrium. Any object which remains at rest must be in equilibrium because if there were any net force or torque, then the object would accelerate and start moving, rotating, or both. Show
Does equilibrium have an equation? Free body diagrams for two suspended masses When solving equilibrium problems we are often trying to find one or more unknown forces using the fact that the net force and torque must be zero. A free body diagram is essential or you will not be able to keep track of the directions and positions of the forces. In the diagram on the right two iron balls are hung from rods. The free body diagrams include the weight of each ball and the forces from the rods.
Rotational equilibrium and torque
(2.10)  Forces:  F1 + F2 + F3 + ... = 0
  Torques:  τ1 + τ2 + τ3 + ... = 0
F  = force (N)
τ  = torque (Nm)
Equilibrium
How do I solve equilibrium problems? The key to solving equilibrium problems is drawing an accurate free-body diagram that includes all forces acting on the object in about the right locations. Also important is to assign positive and negative directions and be consistent about signs when writing the force equation of equilibrium.
  1. Include forces from any ropes, attached supports, weight, and normal forces wherever the object touches another object, or rests on a surface.
  2. Write the equation of equilibrium by setting the net force to zero for each direction. Many problems have forces that act in two perpendicular directions such as up-down (y) and left-right (x).
  3. Each direction can be solved for one unknown force. If there are more unknown forces, you need to consider torques as well so read the next page
An example equilibrium problem In many problems there may be a symmetry which cause some forces to be equal. In the example below, the forces from each rope must be equal because the acrobat is in the center, or symmetric position. That means these forces are assigned the same variable name, F, instead of separate names such as F1 and F2

Test your knowledge If, while moving at 9 m/s, a box experiences a friction force of 40 N, how much force do you need to apply to the box for it to be in equilibrium?
  1. 0 N
  2. 20 N
  3. 40 N
  4. 80 N
    5. Show
Solved Problem 2.4: Equilibrium
An acrobat is hanging motionless from two rings. If the acrobat has a mass of 65 kg, what is the force in each of the ropes?

Asked: The force in the ropes Free body diagram for acrobat
Given: The acrobat is 65 kg and is motionless in the center, between 2 ropes.
Relationships: Fnet = 0, W = mg
Solution: 2F - mg = 0 therefore F = mg/2 = (65 kg)(9.8 m/s2) / 2 = 318.5 kg m/s2 = 318.5 N

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