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
2.2 - Forces and Newton's laws
An underlying belief in physics is that every change that occurs, which are the effects we observe, has a cause. For example, the downward acceleration of a falling ball is caused by the force of Earth's gravity. Section 1 provided models for describing motion using position, velocity, and acceleration. This section investigates the causes which affect motion, starting with the idea of force. Force is the cause that effects changes in motion.
Forces
What is a force? Conceptual definition of force We all have an intuitive sense of the meaning of force. Force is an action represented by words such as push or pull which you might use to describe forces you might exert or which might be exerted on you. In physics, the concept of force is defined with strength (or magnitude), direction, and units.
Definition of the newton, the unit of force
How is force measured? The SI unit of force is the newton. One newton (N) is about the weight of a cell phone—a rather small force. You can easily exert a force of a hundred newtons or more with one arm. Technically, one newton is the force required to change an object's velocity by one meter per second in one second. This is where the newton gets its fundamental units—one newton is the force required to accelerate one kilogram at one meter per second per second (1 N = 1 kg m/s2).
The connection to acceleration Forces are connected to acceleration − to changes in motion. An object at rest will not changes its velocity unless there is a force applied that causes the object to accelerate. Less obvious is that an object already moving will keep moving until a force is applied to change its velocity. Picture an ice skater on ideal frictionless ice. The skater could coast forever if no forces are applied to slow down or turn.
What are pounds? Weight is a force In your everyday life force is often measured in pounds. One pound is equal to 4.448 newtons. If you weigh 100 pounds you also weigh 444.8 newtons. In the grocery store, pounds are divided into ounces. One pound equals 16 ounces therefore, one newton is a just a bit less than 4 ounces. Many practical problems in life, such as buying a rope or building a house will require you to understand forces in pounds. For most physics problems, however, the relationship between force and motion makes it much more convenient to use newtons.
Test your knowledge An object at rest will not change it's velocity unless there is a _________ applied that causes the object to accelerate.
  1. speed
  2. force
  3. distance
  4. time
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