Chapter 2
Physics
Everything there is to know about physics – excerpted from “Everything you need to know about school” in the September 16, 2008 edition of the Seattle periodical The Stranger :
If stu is still, it doesn’t like to move; if stu is moving, it doesn’t like to stop. The more stu you are trying to move, the more you need to push it to speed it up. When you push on stu , it pushes back on you. Stu likes other stu , from a distance at least. Stu likes becoming more chaotic, but cannot be created or destroyed; stu can be rearranged. (All that is Isaac Newton.) Energy, like stu , cannot be created or destroyed, only changed from one kind to another. Energy can be stored in movement, bonds between stu , and many other places. Changes in how energy is stored allow us to do things – like bake, drive, get up tall buildings, and kill each other. (That’s Sadi Carnot.) Also, stu is energy. Stu is a lot of energy. (That’s Albert Einstein.) Compress plutonium with explosives and the atoms fission, releasing the energy stored in stu . When the energy is released in downtown Nagasaki, you kill about 40,000 people right away and another 40,000 over time. (Thanks, Enrico Fermi!)
There is a lot of truth to this quote, despite its sarcastic tone. Physics really is the study of matter and energy, and of the laws governing the interactions of both. The movement of matter largely follows the laws described by Isaac Newton (so much so that this category of physics is often referred to as Newtonian physics). The flow and transformation of energy is a category called thermodynamics. Albert Einstein went on to equate matter with energy through his famous equation E = mc2 which helped usher in the age of nuclear physics.
Physics is important to the study of industrial instrumentation because its laws describe and explain so many applications of measurement and control. As a teacher, one of the things I like to tell students is that their chosen field is really nothing more than applied physics. Granted, there aren’t too many applications of macroscopic projectile motion in industrial measurement and control, but on a microscopic level this is what all moving fluids are: an extremely large collection of independent molecules obeying Newtonian laws of motion. Physics is the study of stu , while instrumentation is the discipline of measuring and controlling stu . The better you understand stu , the better you will be able to measure and control that stu . Enjoy!
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CHAPTER 2. PHYSICS |
2.1Terms and Definitions
Mass (m) is the opposition an object has to acceleration (changes in velocity). Weight is the force (F ) imposed on a mass by a gravitational field. Mass is an intrinsic property of an object, regardless of the environment. Weight, on the other hand, depends on the strength of the gravitational field in which the object resides. A 20 kilogram slug of metal has the exact same mass whether it rests on Earth, or in the zero-gravity environment of outer space, or on the surface of the planet Jupiter. However, the weight of that mass depends on gravity: zero weight in outer space (where there is no gravity to act upon it), some weight on Earth, and a much greater amount of weight on the planet Jupiter (due to the much stronger gravitational field of that planet).
Since mass is the opposition of an object to changes in velocity (acceleration), it stands to reason force, mass, and acceleration for any particular object are directly related to one another:
F = ma
Where,
F = Force in newtons (metric) or pounds (British) m = Mass in kilograms (metric) or slugs (British)
a = Acceleration in meters per second squared (metric) or feet per second squared (British)
If the force in question is the weight of the object, then the acceleration (a) in question is the acceleration constant of the gravitational field where the object resides. For Earth at sea level,
agravity is approximately 9.81 meters per second squared, or 32.2 feet per second squared. Earth’s gravitational acceleration constant is usually represented in equations by the variable letter g instead
of the more generic a.
Since acceleration is nothing more than the rate of velocity change with respect to time, the force/mass equation may be expressed using the calculus notation of the first derivative:
F = m dvdt
Where,
F = Force in newtons (metric) or pounds (British) m = Mass in kilograms (metric) or slugs (British)
v = Velocity in meters per second (metric) or feet per second (British) t = Time in seconds
Since velocity is nothing more than the rate of position change with respect to time, the force/mass equation may be expressed using the calculus notation of the second derivative (acceleration being the derivative of velocity, which in turn is the derivative of position):
d2x F = m dt2
Where,
F = Force in newtons (metric) or pounds (British) m = Mass in kilograms (metric) or slugs (British) x = Position in meters (metric) or feet (British)
t = Time in seconds