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Lecture 21
March 3, 1997
Physics 1000
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Reading Assignment:
Chapter 7of Tippens.
We complete our discussion
of graphs of position and velocity under constant acceleration versus time.
In particular we concentrate on the last subject of the prevous lecture,
a graph of v(t) and x(t) for a falling
body in the absence of air resistance. These graphs can be summarized
in words in at least the following ways:
- Velocity versus time is
linear for all three balls. All three velocity curves are parallel to one
another since they have the same slope: -g.
- For the ball thrown upward,
the sign of velocity changes from positive (upward) to negative (downward)
after about 1s -- that's when it reaches its apex. At its apex, this ball's
velocity is instantaneously zero.
- Position versus time is
parabolic.
- The ball thrown downward
will land first, followed by the dropped ball and then the ball thrown
upward.
- To determine the velocity
of each ball when it hits the ground, you need to read the time when the
ball hits the ground off the x(t) curve and find the velocity for each
of those times. The ball thrown downward will land with a slightly greater
speed than the ball dropped. However, the ball thrown upward will land
with the same speed as the ball thrown downward.
- The local slope of the the
x(t) curve is just the velocity of the ball at the value of time at which
the slope was computed.
- And so on and so forth.
Try to look at these graphs and find other sentences that describe the
physics that they represent.
We conclude our discussion
of graphs of a(t), v(t), and x(t) with a couple of summary slides: summary_1,
summary_2
Link for the Day:
The
Physics Classroom
(Some Physics Classes
on the Web)
Questions or comments should
be addressed to Mike Ritzwoller at ritzwoller@phys-geophys.colorado.edu
Go to Lectures in Physics 1000.
Go to Physics 1000 Home Page.
