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F. I have been using the term “velocity” and avoiding the more common
English word “speed,” because some introductory physics texts define them to
mean different things. They use the word “speed,” and the symbol “s” to mean
the absolute value of the velocity, s=|v|. Although I have thrown in my lot with
the minority of books that don’t emphasize this distinction in technical
vocabulary, there are clearly two different concepts here. Can you think of an
example of a graph of x vs. t in which the object has constant speed, but not
constant velocity.
G. In the graph on the left, describe how the object’s velocity changes.
H. Two physicists duck out of a boring scientific conference to go get beer. On
the way to the bar, they witness an accident in which a pedestrian is injured by
a hit-and-run driver. A criminal trial results, and they must testify. In her
testimony, Dr. Transverz Waive says, “The car was moving along pretty fast, I’d
say the velocity was +40 mi/hr. They saw the old lady too late, and even
though they slammed on the brakes they still hit her before they stopped.
Then they made a U turn and headed off at a velocity of about -20 mi/hr, I’d
say.” Dr. Longitud N.L. Vibrasheun says, “He was really going too fast, maybe
his velocity was -35 or -40 mi/hr. After he hit Mrs. Hapless, he turned around
and left at a velocity of, oh, I’d guess maybe +20 or +25 mi/hr.” Is their
testimony contradictory. Explain.
Discussion question G.
x
t
Section 2.3Graphs of Motion; Velocity.
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