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phy 231
Your 'initial timing 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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6/20 10:29 pm
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Note: The majority of student report taking less than an hour on this experiment, though a few report significantly longer times.
Take reasonable care to get good data in this experiment. Try to do the timing as accurately as possible. Measurements of length, height, etc. should be reasonably accurate (e.g., with a meter stick or ruler you can measure to withing +- 1 millimeter, but it's not necessary to try to determine fractions of a millimeter).
In this experiment you will use the TIMER program, a hardcover book, the toy car that came in your lab materials package (or, if you do not yet have the package, a cylinder or some other object that will roll along the book in a relatively straight line), and a ruler or the equivalent (if you don't have one, note the Rulers link, which is also given on the Assignments page).
The book's cover should be straight and unbent.
The toy car (or other object) should roll fairly smoothly.
Place the book on a flat level tabletop. You will prop one end of the book up a little bit, so that when it is released the object will roll without your assistance, gradually speeding up, from the propped-up end to the lower end. However don't prop the end up too much. It should take at least two seconds for the ball to roll down the length of the book when it is released from rest. For a typical book, a stack of two or three quarters placed under one end works well.
Using the TIMER program determine how long it takes the ball to roll from one end of the ramp to the other, when released from rest. Once you've got the book set up, it takes only a few seconds to do a timing, so it won't take you long to time the object's motion at least three times.
Determine how far the object travels as it rolls from its initial position (where you first click the timer) to its final position (where you click at the end of the interval). This will probably be a bit less than the length of the book, due to the length of the object itself.
Determine how much higher one end of the book was than the other, and how far it is from the supports (e.g., the stack of quarters, or whatever you used to support one end) to the end of the book which rests on the table.
Then reverse the direction of the book on the tabletop, rotating the book an its supports (e.g., the stack of quarters) 180 degrees so that the ball will roll in exactly the opposite direction. Repeat your measurements.
In the box below describe your setup, being as specific as possible about the book used (title, ISBN) and the object being used (e.g., a can of vegetables (full or empty; should be specified) or a jar (again full or empty); anything round and smooth that will upon release roll fairly slowly down the incline), and what you used to prop the object up (be as specific as possible). Also describe how well the object rolled--did it roll smoothly, did it speed up and slow down, did it roll in a straight line or did its direction change somewhat?
your brief discussion/description/explanation:
I used my Physics book for this class, ISBN 978-0-321-50121-9 and the Matchbox car sent in my lab material. I used four quarters to prop the Physics book up.
The car rolled smoothly, speeding up as it reached the finish line. It rolled in a straight line.
I reversed the direction of the ramp 180 degrees. In this direction the car also sped up as it rolled and in a straight line.
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In the space indicated below report your data. State exactly what was measured, how it was measured, how accurately you believe it was measured and of course what the measurements were. Try to organize your report so the reader can easily scan your data and identify any patterns or trends.
your brief discussion/description/explanation:
I set up the book, one end resting on the floor. The other end was raised 8 mm by 4 quarters which was approximately 29 cm from the resting end of the book. The ramp from starting point to finish line was 33.3 cm in length. I placed the car, back wheels at the starting point. I released the car three different times and measured 2.0625, 2.21875, 2.125 seconds at the finish line (back wheel crossing).
I reversed the direction of the ramp 180 degrees. I repeated the experiment and measured 3.773438, 4.375, and 4.015625 seconds when the car crossed the finish line. I found it odd that the car rolled slower in the opposite direction. In this direction the car also sped up as it rolled and in a straight line.
The measurement of each distance is accurate to the nearest tenth of a centimeter. The time measured is accurate if stopping and starting the timer corresponded exactly to the start and finish of the car, which was accomplished as close as possible.
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Using your data determine how fast the object was moving, on the average, as it rolled down the incline. Estimate how accurately you believe you were able to determine the object's average speed, and give the best reasons you can for your estimate of the accuracy.
your brief discussion/description/explanation:
The average speed is calculated by total distance/total time for car to travel the total distance, the first three trial runs resulted in
16.2 cm/sec
15.0 cm/sec
16.7 cm/sec
The average speed of the three trial runs resulted in 16.0 cm/sec
Turning the car and ramp 180 degrees and repeating three trial runs resulted in
8.8 cm/sec
7.6 cm/sec
8.2 cm/sec
The average speed of the three trial runs at 180 degree turn resulted in 8.2 cm/sec
The calculations are only as accurate as the least significant digit in the calculation. With the tools I used the least accurate measurement is the ruler. I could only measure distance to the nearest tenth of a centimeter. In this experiment the distance of the ramp had 3 significant digits in 33.3 cm. Therefore, I rounded all my calculations to the nearest tenth.
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How fast was the object moving at the beginning of the timed interval?
According to your previous calculation, what was its average speed during this interval?
Do you think the object, when it reached the lower end of the book, was moving at a speed greater or less than the average speed you calculated?
your brief discussion/description/explanation:
The car has an initial speed of 0 cm/sec, at the beginning of the time interval. The average speed calculated in the experiment was 16.0 cm/sec in one direction and 8.2 cm/sec in the opposite direction.
The car was moving at a speed greater than the average speed when it reached the lower end of the book and the finish line. This was noticeable by the naked eye.
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List the following in order, from least to greatest. Indicate 'ties': The object's initial speed, its final speed, its average speed, and the change in its speed as it rolled from one end of the book to the other.
your brief discussion/description/explanation:
From least to greatest: the cars initial speed, tie - the change in its speed as it rolled from one end of the book to the other and its average speed, its final speed
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Devise and conduct an experiment to determine whether or not the object is speeding up as it rolls down the incline. If you have set the experiment up as indicated, it should seem pretty obvious that the object is in fact speeding up. But figure out a way to use actual measurements to support your belief.
Explain how you designed and conducted your experiment, give your data and explain how your data support your conclusions.
your brief discussion/description/explanation:
To determine whether or not the car is speeding up as it rolls down the incline I set up the following experiment:
The book was placed with one end resting on the table and the other end raised with 4 quarters. The quarters raise the book 8 mm on that end. The length of the ramp the car travels is 33.3 cm long. I marked a distance of 9.6 cm from the start of the ramp and used the timer to measure the time it takes to pass 9.6 cm. After three trials the time intervals resulted: .765625, .875, and .828125. This yields respectfully, 12.5 cm/sec, 11.0 cm/sec, 11.6 cm/sec average speeds. The overall average speed for the car to reach the 9.6 mark is 11.7 cm/sec.
I released the car again, this time measuring distance and time at the end of the ramp. At the end of the ramp I again used the timer to determine how long it takes the car to cover the last 10 cm of the ramp. After three trial runs the intervals resulted in .3125, .3125, .25. This yields 32 cm/sec, 40 cm/sec respectfully.
The results showed a significant difference in speed at the beginning than at the end.
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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?
1.5 hours
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You may also include optional comments and/or questions.
I found it very odd that the results for the average speed after turning the book/ramp 180 degrees were significantly slower. To verify this I tried two different books and used a small steel ball and also found after I turned the ramp each time, it rolled slower. The ball rolled to the edge and down the crease of the book and the car several times, just sat there, not wanting to move. I also tried moving the experiment to the floor instead of the table and still got the same results - the car moved slower when the ramp was turned 180 degrees and it was moving in the opposite direction. I thought it should have been the same speed.
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@& Very good.
You are dealing with a fairly small slope to start with. Almost all floors are at least slightly off level, and with the small slope of the ramp this is enough to make a significant difference in the times. Nearly all students observe this.
The other factor is that everyone's timing is at least slightly biased, and sometimes the bias is greater in one direction than in the other.
Nearly all students report discrepancies when the direction is reversed.*@