BEGINNING OUR DISCUSSION OF MOTION
When we meet again on 21 March, we will begin our discussion of motion. By March 21, I want you to read through chapters 2 and 3 in the
text, and work on the problems I will give you below.
These chapters will introduce you to the basic concepts of motion. One of the most important aspects of this material that you will
face, and certainly your future students will face, will be the need to know the proper meaning of terms and use them appropriately.
Part of the difficulty in this is that the terms we will encounter, speed, velocity, and acceleration are
all terms we know from everyday English, and the temptation will be great to rely on our "everday" understanding of these words rather
than the precise scientific definitions we will study.
Please read the text carefully to realize that speed and velocity are not interchangeable terms, they are certainly
related, but are not equivalent. Speed measures only the rate or magnitude of motion, whereas velocity is a
description of both rate and direction of travel. So, a statement like "30 mi/hr" is a description of speed, whereas the phrase
"the winds are from the north at 25 mi/hr" is a statement of velocity, since it combines both rate of motion and direction of
Speed and velocity are examples of classes of objects or concepts known as scalars and vectors. Scalars are objects which
are described by magnitude alone (i.e., a number without direction describes them fully) whereas vectors are described by both magnitude
and direction. See if you can think of other examples of scalars and vectors.
One of the key concepts we will study in Chapters 2 and 3 will be acceleration. The everyday meaning of this term is clear, it
is almost always used to mean "speed up", as in "this car will go from 0 to 60 in 3 secs flat". This is actually a fair description of
one type of acceleration, since it describes how much the speed changes and the time in which that change occurs, but it is important to
note that there are many types of acceleration. In physics, acceleration refers to any change of motion, and this includes any
change in speed (either speeding up or slowing down) or any change in direction. Consider driving a car; suppose you wanted to make the
car travel in a circle, what would you do to cause this to occur? Suppose you wanted to keep the car travelling in the same circle for
five complete laps, imagine what you would have to do to accomplish that. Now, suppose at the end of the fifth lap, you let the
steering wheel go back to its original position. What would be the motion of the car then?
We will find that acceleration is a key conept in understanding forces (the subject of chapters 4-6). We will find that forces tend to
cause accelerations, and the greater the force acting on an object, the greater its acceleration can be.
Chapters two and three introduce us to some of the simple calculations we will do in this course. In reviewing the calculations done in
the book, please note how important it is to include all units in your calculations. For instance, in the example given above (the car
going from zero to sixty in three seconds flat...), we can calculate the acceleration of the object, as long as we tighten up the
language and include appropriate units.
In everyday spoken English, the phrase "...zero to sixty..." almost certainly refers to the speed measured in miles/hour. Given this,
we can use the simple definition of acceleration given on pp. 15-16 of the text:acceleration = change in speed/time interval
acceleration = (60 mi/hr - 0 mi/hr)/3 secs = 20 mi/hr/sec
This calculation and the resulting answer are noteworthy for a few reasons. First, notice how units are treated; it is clear that the
calculation calls for a division, and there should be no anxiety that 60/3=20. However, notice that the division also extends to the
units; dividing the units in the numerator (mi/hr) by the units in the denominator (sec) gives you the final result of mi/hr/sec (you
would say this as 'miles per hour per second'). What does this strange looking unit mean? It means that the car is accelerating, i.e.,
changing its motion (and in this particular case, its speed) at the rate of 20 mi/hr for every second in the time interval under
Please read the text as assigned and for the first day back, 21 March, please turn in complete answers to the following. (In doing
calculations, you must show all work and how you arrive at an answer, not merely present the answer.)
1. Suppose a car travels on a perfectly circular track at a constant speed of 20 mi/hr. Is the car accelerating? Explain why or why
2. Do numbers 27, 28, 33, 34, 42 from pp. 26-27 of the text.
3. Read no. 52/p. 27 in the activities section, and try to do this with a friend. Be sure to write down your results. (If you can't get
this done before class, that's fine also.)
4. Numbers 19, 21, 29 and 30 from p. 41 in the text.
Please do not hesitate to contact me if you have any questions.
Enjoy the rest of your break, and I look forward to seeing you on the 21st.
David B. Slavsky
Loyola University Chicago
Cudahy Science Hall, Rm. 404
1032 W. Sheridan Rd.,
Chicago, IL 60660
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