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
Weight
What is weight? Weight is a force that results from the action of gravity on mass. At Earth’s surface, gravity attracts all masses downward with a force of 9.8 newtons per kilogram. This gravitational force is called weight. To calculate an object's weight, multiply its mass by the strength of gravity in N/kg. For example, a 10 kg object has a weight of 98 N = 9.8 N/kg × 10 kg.
(2.7) F w =mg
Fw  = Weight force (N)
m  = mass (kg)
g  = Strength of gravity = 9.8 N/kg at Earth's surface
Weight
The value of “g” The value of 9.8 N/kg is used so often it has its own symbol, g. When you see “g” in an equation such as F = mg , substitute the strength of gravity at Earth’s surface, which is g = 9.8 N/kg. You may sometimes see g with different units, such as g = 9.8 m/s2. This is an equivalent definition because 1 N of force is defined as the force that causes a 1 kg object to accelerate at 1 m/s2.
Are weight and mass the same thing? Weight is not mass. Mass is measured in kilograms and grams. Weight is measured in newtons or pounds. The source of confusion is that we use weight to measure mass! When you place an object on an electronic balance, the sensor in the balance measures the force of gravity acting on the object. Since this force is proportional to mass, a calculation done inside the balance divides the force by 9.8 N/kg to read mass in kilograms, or by 0.0098 N/g to read mass in grams. However, a balance calibrated to work on Earth will not be accurate on the Moon or anyplace where gravity is not exactly the same as it is on Earth. In the extreme case far from any planet or star, an object will still have its mass, but may be weightless because of the absence of gravity.
How do I calculate weight? In the SI system, equation (2.7) relates weight in newtons to mass in kilograms. To use the equation the units of each value must be in the same system—if force is in newtons, and g is in N/kg, then mass must be in kilograms. Many physics problems will require that you convert from every-day units, such as pounds, to units that work with an equation in SI units, such as newtons. Do the conversions before substituting any values into the equation to calculate a result.Solving for mass when given weight in pounds
Force conversion factors and values for the force of gravity at the surface of Earth
Test your knowledge How many pounds of force does a man with a mass of 80kg exert by standing on the Moon (g = 1.6 N/kg)?
  1. 128 N
  2. 64 N
  3. 784 N
  4. 50 N
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