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course Phy 242
Use a plastic soft-drink bottle and the TIMER program, as well as two short open tubes from the Initial Materials package (sent for free when you submitted your address, according to the email request made previously). If you don't yet have those materials you can improvise with a drinking straw and a plastic soft-drink bottle.
The experiment just asks fora few predictions and some data, and once set up takes only a few minutes to run. Don't go to a lot of trouble trying to be overly precise, but do try to be reasonably accurate (e.g., +- a second on timing, +- a few millimeters on positions of the three lines, +- a centimeter or two on the horizontal range of the stream). You'll be refining things and taking a little more care on followup trials. You should easily be able to get this done before Monday.
Do spend some time on the problems and submit what you are able to get prior to Monday's class. You'll learn a lot more in class if you do so. We'll be going over the questions in class.
You are going to do the following (don't do it yet; predict first what will happen):
• Punch a hole the side of the bottle, near the bottom.
• Insert a piece of tubing into the hole to direct the flow of water from the bottle.
• Fill the bottle with water.
• Set it on a tabletop or on the counter next to the sink (or on the ledge of a bathtup, etc.--any setup will do as long as the bottle rests on a level surface, and the escaping stream has at least 15 cm to fall before it hits something).
• Let the water run out, with the stream falling freely to the surface below.
What do you expect will happen? Answer the following:
Will the water stream tend to travel longer and longer distances before striking the surface below, shorter and shorter distances, or will the distance tend to increase at times and decrease at times? Why do you think the distance traveled by the stream will behave as you say?
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I believe that the distance the water flows will decrease over time because the amount of force that pushes the water out will decrease as the water level decreases, thus decreasing the acceleration, velocity, and position the water will spurt.
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Will the speed of the escaping water increase, decrease, or sometimes increase and sometimes decrease? Explain your thinking.
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As previously mentioned, with the decrease of force being applied to the water as it exits, the water will escape more slowly.
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Clearly the water level will decrease. Will it decrease more and more quickly, more and more slowly, or sometimes more quickly and sometimes more slowly? Explain your thinking.
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The water will escape at a decreasing speed. If the water is leaving the bottle slower, then the level will be decreasing slower as well.
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Now set up and prepare to take some measurements:
Punch or cut a hole about 1/8 inch (.3 cm) in diameter in the side of the bottle, at a point where the side of the bottle is vertical, within a few centimeters of the bottom of the bottle. If you cut the hole, a triangle about 1/8 inch on a side is about right.
Your initial materials will include short open pieces of tubing, one whose diameter is the same as that of the 'cap' on the bottlecap and tube, the other having the same diameter as the tube. Insert the larger of these pieces into the hole. The piece should fit fairly tightly in the hole, so when the bottle is filled much more of the water that flows from the bottle will flow through the tube rather than around it. You can test this by filling the bottle, placing your thumb over the end of the tube, and seeing how much water leaks out. Then move your thumb and verify that the water flows out much more rapidly.
Mark three points on the bottle, one at the top of the cylindrical section of the bottle, one halfway between the first mark and the hole, and one halfway between the second mark and the hole. Measure the three distances, relative the the hole, with reasonable accuracy.
You will set the bottle it a vertical position and release your thumb. You will time the fall of the water level, reporting the clock times at which the water reaches the first mark, and each subsequent mark.
You will report the vertical positions of the three lines, relative to the hole, and the observed clock times. Your data will be used to determine the duration of each of three intervals:
• During the first interval the water level falls from the highest mark to the second-highest.
• During the second interval the water level falls from the second mark to the third.
• During the third interval the water level falls from the third mark to level of the hole. The water will be considered to have reached the level of the hole when it starts falling from the bottle in separate drops rather than a stream or a continuous series of drops.
Before you actually perform the experiment make some additional predictions:
Which of the four intervals will last the longest, and which will be the shortest?
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I predict that the last one will last the longest and the first will last the shortest. There are only 3 intervals..
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List the predicated intervals in order, from the longest to the shortest, and explain your thinking:
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Third
Second
First
As the water level decreases in the bottle, so does the pressure that the water applies on the exit. This means that the water is being pushed out with less force. As the bottle empties, the water will take longer and longer to fall out but the rate of change will be slower so the third interval would take the longest.
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Did you predict the order of the four intervals correctly?
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No, The first interval was the longest and the last interval was the shortest. I was completely wrong.
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Now run your first trial. (You will also run a second trial, in which the short piece of thinner tubing is inserted into the larger piece to narrow the flow).
Report your data, and explain what it means. If you used the TIMER include a copy of the display of times (you can just copy and paste the display into a document).
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Mark 1: started at zero
Mark 2: reached after 10.5 seconds
Mark 3: reached after 11.2 seconds
Dripping: reached at 13.7 seconds
This data suggests that the force that is pushing the water out of the straw is relatively constant until there is hardly any water above the height of the straw.
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Determine, as accurately as you can using a clock or watch with a second hand, the clock times at which the water reaches the first mark, the second, the third and the clock time at which the flow from the hole reduces to the point where it leaves the hole in distinct drops.
Run your second trial, in which the short piece of thinner tubing is inserted into the larger piece to narrow the flow.
Report your data, and explain what it means. If you used the TIMER include a copy of the display of times (you can just copy and paste the display into a document).
Mark 1: started at t=0
Mark 2: reached after 12.0seconds
Mark 3: reached after 13.2 seconds (1.2 seconds after the second mark was passed)
Dripping:15.1 seconds after starting, the water was dripping out of the straw.
This data suggests the same as above except that using the skinnier straw, the water was forced to empty a bit slower.
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Determine, as accurately as you can using a clock or watch with a second hand, the clock times at which the water reaches the first mark, the second, the third and the clock time at which the flow from the hole reduces to the point where it leaves the hole in distinct drops.
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&#This looks good. Let me know if you have any questions. &#