phy202
Your 'flow experiment' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
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** Is flow rate increasing, decreasing, etc.? **
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The picture below shows a graduated cylinder containing water, with dark coloring (actually a soft drink). Water is
flowing out of the cylinder through a short thin tube in the side of the cylinder. The dark stream is not obvious but
it can be seen against the brick background.
You will use a similar graduated cylinder, which is included in your lab kit, in this experiment. If you do not yet
have the kit, then you may substitute a soft-drink bottle. Click here for instructions for using the soft-drink bottle.
In this experiment we will observe how the depth of water changes with clock time.
In the three pictures below the stream is shown at approximately equal time intervals. The stream is most easily found
by looking for a series of droplets, with the sidewalk as background.
Based on your knowledge of physics, answer the following, and do your best to justify your answers with physical
reasoning and insight:
As water flows from the cylinder, would you expect the rate of flow to increase, decrease or remain the same as water
flows from the cylinder?
:I would expect it to decrease because of less water pushing down on it.
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As water flows out of the cylinder, an imaginary buoy floating on the water surface in the cylinder would descend.
Would you expect the velocity of the water surface and hence of the buoy to increase, decrease or remain the same?
:decrease.
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How would the velocity of the water surface, the velocity of the exiting water, the diameter of the cylinder and the
diameter of the hole be interrelated? More specifically how could you determine the velocity of the water surface from
the values of the other quantities?
:they are linking because as less water is in the cylinder, there is less pressure pushing the water out. possibly how
far the water travels when it leaves the cylinder?
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The water exiting the hole has been accelerated, since its exit velocity is clearly different than the velocity it had
in the cylinder.
Explain how we know that a change in velocity implies the action of a force?
:because objects remain in motion, the same motion, until they are acted upon by an outside force. there has to be
another force to accelerate the water.
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What do you think is the nature of the force that accelerates the water from inside the cylinder to the outside of the
outflow hole?
:it is the weight of the above water due to gravity.
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From the pictures, answer the following and justify your answers, or explain in detail how you might answer the
questions if the pictures were clearer:
Does the depth seem to be changing at a regular rate, at a faster and faster rate, or at a slower and slower rate?
:at a slower and slower rate.
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What do you think a graph of depth vs. time would look like?
:A curved line getting less and less steep.
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Does the horizontal distance (the distance to the right, ignoring the up and down distance) traveled by the stream
increase or decrease as time goes on?
:it decreases as time goes on.
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Does this distance change at an increasing, decreasing or steady rate?
I think it changes at a decreasing rate.
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What do you think a graph of this horizontal distance vs. time would look like? Describe in the language of the
Describing Graphs exercise.
:I think it would be decreasing at a decreasing rate.
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You can easily perform this experiment in a few minutes using the graduated cylinder that came with your kit. If you
don't yet have the lab materials, see the end of this document for instructions an alternative setup using a soft-drink
bottle instead of the graduated cylinder.
Setup of the experiment is easy. You will need to set it up near your computer, so you can use a timing program that
runs on the computer. The cylinder will be set on the edge of a desk or tabletop, and you will need a container (e.g.,
a bucket or trash can) to catch the water that flows out of the cylinder. You might also want to use a couple of towels
to prevent damage to furniture, because the cylinder will leak a little bit around the holes into which the tubes are
inserted.
Your kit included pieces of 1/4-inch and 1/8-inch tubing. The 1/8-inch tubing fits inside the 1/4-inch tubing, which in
turn fits inside the two holes drilled into the sides of the graduated cylinder.
Fit a short piece of 1/8-inch tubing inside a short piece of 1/4-inch tubing, and insert this combination into the lower
of the two holes in the cylinder. If the only pieces of 1/4-inch tubing you have available are sealed, you can cut off
a short section of the unsealed part and use it; however don't cut off more than about half of the unsealed part--be
sure the sealed piece that remains has enough unsealed length left to insert and securely 'cap off' a piece of 1/4-inch
tubing.
Your kit also includes two pieces of 1/8-inch tubing inside pieces of 1/4-inch tubing, with one end of the 1/8-inch
tubing sealed. Place one of these pieces inside the upper hole in the side of the cylinder, to seal it.
While holding a finger against the lower tube to prevent water from flowing out, fill the cylinder to the top mark (this
will be the 250 milliliter mark).
Remove your thumb from the tube at the same instant you click the mouse to trigger the TIMER program.
The cylinder is marked at small intervals of 2 milliliters, and also at larger intervals of 20 milliliters. Each time
the water surface in the cylinder passes one of the 'large-interval' marks, click the TIMER.
When the water surface reaches the level of the outflow hole, water will start dripping rather than flowing continuously
through the tube. The first time the water drips, click the TIMER. This will be your final clock time.
We will use 'clock time' to refer to the time since the first click, when you released your thumb from the tube and
allowed the water to begin flowing.
The clock time at which you removed your thumb will therefore be t = 0.
Run the experiment, and copy and paste the contents of the TIMER program below:
2.265625
2.292969
2.726563
2.761719
2.917969
3.058594
3.683594
3.695313
4.820313
5.039063
9.015625
5.617188
The TIMER program has three columns, but I can tell what this one means, so no problem.
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Measure the large marks on the side of the cylinder, relative to the height of the outflow tube. Put the vertical
distance from the center of the outflow tube to each large mark in the box below, from smallest to largest distance.
Put one distance on each line.
1.6
3.5
5.4
7.3
9.2
11.0
12.8
14.6
16.3
17.9
19.2
20.8
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Now make a table of the position of the water surface vs. clock time. The water surface positions will be the positions
of the large marks on the cylinder relative to the outflow position (i.e., the distances you measured in the preceding
question) and the clock times will as specified above (the clock time at the first position will be 0). Enter 1 line
for each event, and put clock time first, position second, with a comma between.
For example, if the first mark is 25.4 cm above the outflow position and the second is 22.1 cm above that position, and
water reached the second mark 2.45 seconds after release, then the first two lines of your data table will be
0, 25.4
2.45, 22.1
If it took another 3.05 seconds to reach the third mark at 19.0 cm then the third line of your data table would be
5.50, 19.0
Note that it would NOT be 3.05, 19.0. 3.05 seconds is a time interval, not a clock time. Again, be sure that you
understand that clock times represent the times that would show on a running clock.
The second column of your TIMER output gives clock times (though that clock probably doesn't read zero on your first
click), the third column gives time intervals. The clock times requested here are those for a clock which starts at 0
at the instant the water begins to flow; this requires an easy and obvious modification of your TIMER's clock times.
For example if your TIMER reported clock times of 223, 225.45, 228.50 these would be converted to 0, 2.45 and 5.50 (just
subtract the initial 223 from each), and these would be the times on a clock which reads 0 at the instant of the first
event.
Do not make the common error of reporting the time intervals (third column of the TIMER output) as clock times. Time
intervals are the intervals between clicks; these are not clock times.
:
0, 20.8
2.266, 19.2
4.559, 17.9
7.285, 16.3
10.047, 14.6
12.965, 12.8
16.023, 11.0
19.707, 9.2
23.402, 7.3
28.223, 5.4
33.262, 3.5
42.277, 1.6
47.895, 0
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You data could be put into the following format:
clock time (in seconds, measured from first reading) Depth of water (in centimeters, measured from the hole)
0
14
10
10
20
7
etc.
etc.
Your numbers will of course differ from those on the table.
The following questions were posed above. Do your data support or contradict the answers you gave above?
Is the depth changing at a regular rate, at a faster and faster rate, or at a slower and slower rate?
: they support the answer i gave above.
the depth is changing at a slower and slower rate.
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Sketch a graph of depth vs. clock time (remember that the convention is y vs. x; the quantity in front of the 'vs.' goes
on the vertical axis, the quantity after the 'vs.' on the horizontal axis). You may if you wish print out and use the
grid below.
Describe your graph in the language of the Describing Graphs exercise.
th graph is decreasing at a decreasing rate. it gets less and less steep as the time goes on.
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caution: Be sure you didn't make the common mistake of putting time intervals into the first column; you should put in
clock times. If you made that error you still have time to correct it. If you aren't sure you are welcome to submit
your work to this point in order to verify that you really have clock times and not time intervals
Now analyze the motion of the water surface:
For each time interval, find the average velocity of the water surface.
Explain how you obtained your average velocities, and list them:
1 .704 cm/sec
2 .741 cm/sec
3 .623 cm/sec
4 .615 cm/sec
5 .583 cm/sec
6 .556 cm/sec
7 .461 cm/sec
8 .460 cm/sec
9 .353 cm/sec
10 .337 cm/sec
11 .189 cm/sec
12 .303 cm/sec
I obtained the velocities by taking the distance traveled for each interval and dividing it by the time taken to travel
that distance. (note: the last time interval was only from the last mark to the hole in the cylinder so it had less time
and distance to travel).
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Assume that this average velocity occurs at the midpoint of the corresponding time interval.
What are the clock times at the midpoints of your time intervals, and how did you obtain them? (Give one midpoint for
each time interval; note that it is midpoint clock time that is being requested, not just half of the time interval.
The midpoint clock time is what the clock would read halfway through the interval. Again be sure you haven't confused
clock times with time intervals. Do not make the common mistake of reporting half of the time interval, i.e., half the
number in the third column of the TIMER's output):
1 1.13
2 3.41
3 5.922
4 8.667
5 11.506
6 14.494
7 17.865
8 21.555
9 25.813
10 30.743
11 37.770
12 45.086
I obtained my results by adding the average interval of each time to the ongoing time.
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Make a table of average velocity vs. clock time. The clock time on your table should be the midpoint clock time
calculated above.
Give your table below, giving one average velocity and one clock time in each line. You will have a line for each time
interval, with clock time first, followed by a comma, then the average velocity.
1.13, .704 cm/sec
3.41, .741 cm/sec
5.922, .623 cm/sec
8.667, .615 cm/sec
11.506, .583 cm/sec
14.494, .556 cm/sec
17.865, .461 cm/sec
21.555, .460 cm/sec
25.813, .353 cm/sec
30.743, .337 cm/sec
37.770, .189 cm/sec
45.086, .303 cm/sec
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Sketch a graph of average velocity vs. clock time. Describe your graph, using the language of the Describing Graphs
exercise.
:the graph is decreasing at a decreasing rate. getting less and less steep.
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For each time interval of your average velocity vs. clock time table determine the average acceleration of the water
surface. Explain how you obtained your acceleration values.
.317
.323
.228
.223
.200
.182
.125
.124
.073
.067
.021
.054
i obtained them by divided the average velocity by the time interval again to get seconds^2 for acceleration.
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Make a table of average acceleration vs. clock time, using the clock time at the midpoint of each time interval with the
corresponding acceleration.
Give your table in the box below, giving on each line a midpoint clock time followed by a comma followed by
acceleration.
1 1.13, .317
2 3.41, .323
3 5.922, .228
4 8.667, .223
5 11.506, .200
6 14.494, .182
7 17.865, .125
8 21.555, .124
9 25.813, .073
10 30.743, .067
11 37.770, .021
12 45.086, .054
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Answer two questions below:
Do your data indicate that the acceleration of the water surface is constant, increasing or decreasing, or are your
results inconclusive on this question?
Do you think the acceleration of the water surface is actually constant, increasing or decreasing?
it is decreasing. i think that it is actually decreasing.
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Your instructor is trying to gauge the typical time spent by students on these experiments. Please answer the following
question as accurately as you can, understanding that your answer will be used only for the stated purpose and has no
bearing on your grades:
Approximately how long did it take you to complete this experiment?
3 hours 45 minutes
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&#This looks very good. Let me know if you have any questions. &#