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Physics Terminology 1 . In physics, everyday words that we are familiar with may have special usage and meaning; so we begin our study of physics by learning a few of these terms.
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Physics Terminology 1. In physics, everyday words that we are familiar with may have special usage and meaning; so we begin our study of physics by learning a few of these terms. Quantity: In everyday usage, the word quantity means the amount of a substance; or how much of a substance we have. In addition to this meaning, quantity in physics refers to anything that we can measure or count. For example, time, length, money, mass, weight, are all quantities because we can measure or count them.
Physics Terminology 2. • Measurement:Measurement is the process by which we compare a quantity with a known amount (called a standard) of the same quantity in order to determine how much of the quantity we have. For example, if we want to know the length of our football field, we could lay meter sticks (a meter is our standard for length) along the length of the football field and count how many meter sticks can fit along the length. Of course, this is not practical; so we would use a tape measure which is calibrated in meters to take this measurement.
Physics Terminology 3. As we’ve learned so far, quantities are things that we can count or measure in order to determine the amount or size of the quantity. There are certain quantities however, that knowing the amount or size alone does not give a complete description (or information) of the quantity. For example, imagine we are lost at sea and call the Coast Guard to come to our rescue. We tell the Coast Guard that we are 200 miles off the coast of Tampa because that’s all we know. The Coast Guard will have a hard time finding us because they wouldn’t know which way to go to get to us. In light of this, we classify physics quantities into two categories: A scalar quantityneeds only magnitude or size to completely specify it. Examples: money, time, mass, distance, speed. A vector quantityneeds both magnitude and size to completely specify it. Examples: displacement, velocity, force, acceleration.
Physics Terminology 4. Aside from classifying quantities as vector or scalar quantities, we also classify quantities into two additional categories as follows: A basic quantityis a quantity that cannot be expressed in terms of other quantities. Examples are length, time and mass. A derived quantity is a quantity that can be expressed in terms of the basic quantities. Examples are volume, area, force, density. There are only 7 basic quantities in physics. The vast majority of quantities are derived quantities. In mechanics, we will encounter 3 basic quantities – length, massandtime. In thermal physics, we will encounter one additional basic quantity – temperature; and in electricity and magnetism, we will encounter an additional basic quantity – electric current. (Note: In older physics textbooks, the electriccharge is considered the basic unit instead of the electric current.)
Physics Terminology 5. When we measure quantities, we express the measurements in units. Just as there are basic and derived quantities so are there basic and derived units to go with them. A basic (or base) unit is used to express the measurement of a basic quantity. A derived unit is used to express the measurement of a derived quantity. Worldwide, the quantities (basic and derived) are the same; but the units in which the quantities are expressed used to be (and still are, to a certain extent) different. This created problems for physicists around the globe. To resolve this problem, physicists met in Sèvres, France in 1960 and adopted the SI (Système International d’unités) as the standard for expressing measurements in physics.
Physics Terminology 6. The SI System The SI system is a subset of the metric system. The basic quantities and their basic units in the SI system are shown in the table below: The SI system is often referred to as the mks system (m = meter, k = kilogram, s = seconds).
Physics Terminology 7. The Metric System The metric system is quite easy to use because the units are in multiples of 10. The table below shows some of the commonly used units, their prefixes and their relationship to the standard unit (meter in the table) in powers of 10.
Physics Terminology 8. In the example where we were lost at sea and called the Coast Guard for help we saw that it was necessary that we give the Coast Guard a direction in addition to the distance from shore in order for them to rescue as in a timely manner. Equally important is a reference point from which the distance from shore is measured; which in our case is the Coast of Tampa (quite vague but okay for now). This takes us to the idea of a • Frame of Reference: (In simple terms, it is) a set of axes which serves as a reference for taking measurements. For us in beginning physics, a reference frame is just a convenient set of coordinate axes that we choose as references for taking our measurements.
Kinematic Quantities Distance versus Displacement Constant Speed versus Constant Velocity Average Speed versus Average Velocity Instantaneous Speed versus Instantaneous Velocity Acceleration
Displacement: The straight line distance pointing in the direction from start to finish. Displacement is a vector quantity. Distance: The total distance traveled from start to finish. Distance is ascalar quantity. Questions: In the figure, how do the displacements of cars A (the blue car) and car B (the orange/red) compare? How do the distances traveled by the two cars compare?
Position Vector:The location of an object relative to the origin. Displacement Vector: The location of an object relative to a reference point (not necessarily the origin). Questions: Can you name some position vectors in the figure? What about some displacement vectors? Is it correct to say that the position vector is also a displacement vector? Explain. Position Vector versus Displacement Vector
We can say that a vector is a directed distance. We write vectors in several ways: • We write the starting point followed by the endpoint letters and draw an arrow over them. For example the position vector from the origin to point A is written (pronounced vector OA). The displacement vector from A to C is written as . • The vector is denoted by just one letter with an arrow over it. For example, we can label the distance OA as A (as in diagram) and write as . • In print, a vector is denoted in boldface type, for example OA or A • When we draw vectors, the length of the vector represents its magnitude (or size) and the arrow represents its direction. • Questions: • Name some vectors in the diagram. • Can you say that ACCA, OAAO? Explain why or why not. Vector Notation and Basic Vector Properties A
We see from the previous slide that as far as distance is concerned, OAAO; but for vectors, OAAO. We also note in the figure that if we want to find the total distance traveled by car B, all we need to do is add the segments traveled together. Thus we write (for car B) . We’ve already seen that this total distance traveled by car B is not the same as the displacement of car B. Our question then is: Can vectors be added together? Is there a special rule for adding and subtracting vectors? The answer to both questions is yes. When we add (or subtract) vectors, we get a new vector called the resultant. There are two ways of adding vectors together (described on the next slide): The graphical Method The Analytic or Component method. The negative of a vector is a vector having the same length but opposite direction. Subtracting a vector is the same as adding the negative of the vector being subtracted. Vector Properties and Basic Vector Math A
Average speed: The total distance traveled divided by the time taken to travel that distance. In symbols Average velocity: The displacement divided by the time taken to travel that displacement. In symbols Question: What is the difference between the two formulas above? Average speed and average velocity
