Phy 201
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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6/3 8
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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 University Physics textbook, which was 28 cm long and 6 cm thick (ISBN: 978-0-321-50121-9). The object being rolled down the book was the hot wheels car from the lab kit (Ford Mustang GT). I propped the book up with my ruler, which is about .5 cm thick. The car rolled very smoothly down the book and did not go too fast. Also it stared in pretty much a straight line.
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 measured the time it took for the car to roll, under its own power, from the top of the incline to the bottom. I started the timer program when I released the car and then stopped it when the front wheels touched the bottom edge of the book.
Rolling to the left Time in seconds Rolling to the right Time in seconds
1 1.903 1 2.543
2 1.645 2 2.09
3 1.872 3 1.95
4 1.748 4 2.168
Average 1.792 Average 2.19
Total Average 1.99 seconds
Considering the numbers were rounded after the calculations and the timer program is only accurate to about .1 seconds, I think that there is +/- .2 seconds on all of the times. This is to account for the inaccuracy of the timer program and the uncertainty of rounding the numbers after averaging them.
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:
There is two ways to find the average speed as it rolled down the incline. One is to divide the total distance by the total average time: 22cm / 1.99 seconds = 11.055 cm/s. The other way is to use the two averages, from the car rolling right and left, find the speed going in each direction and then average the two speeds. You get: 22cm / 1.792 sec = 12.2768 cm/s and 22cm / 2.19225 sec = 10.0354 cm/s. Then you average them and get 11.156 cm/s. These two speeds are about .1 seconds away from each other. I would say that the 11.156 cm/s is closer to being accurate because there is one less calculation, and thus chance for rounding, in that method.
The average speeds that were calculated were probably +/- .2 seconds away from the real speed. This is because the timer is not completely accurate and the numbers were rounded after the calculations, which increases the uncertainty of the results.
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:
When the timer was started, the speed of the car was 0 cm/s because the test had just started and had not begun to move yet. However, almost immediately it began to move and gain speed because of gravity.
The average speed of the car during the interval was about 11.156 cm/s.
When the car got to the end of the book it was moving faster than the average speed. This is because the average factors in the very slow motion at the beginning of the test and the very fast at the end and finds the speed in the middle. Therefore, the average speed will not show the maximum speed.
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:
1) Initial speed
2) Average speed
3) Final speed and Change in speed from one end to the other
The final speed and change in speed would be a tie for the fastest because the initial speed was zero. Thus, the change would be up to the final speed with nothing subtracted from it because it started at zero.
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:
Experiment:
Mark the center point of the book (11 cm) with something so that you are able to tell when the car passes that point. Then, start the car down the incline as before and start the timer when the car is released. However, you have to click the timer when the car gets to the middle as well as at the end. Then you can measure the time it took for the car to go down both halves of the book. If the car is picking up speed, the second half will be faster than the first. After you get the times, you can determine the speed of the car on each half by dividing the length of the run by the number of seconds it took to go the distance.
[To do this more accurately, you could film the runs and get more accurate time measurements for the speed of the car.]
Results:
Trial Begin-middle (sec) Middle-end (sec) Speed: begin-middle
1 1.701 .873 7.857 cm/s
2 1.201 .827
3 1.28 .967 Speed: middle-end
4 1.42 .764 19.047 cm/s
Average 1.40 .8578
The car’s speed was much faster during the second half, than it was during the first, which indicates that it was picking up speed as it rolled.
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?
~ 45 minutes
Good.
Reminder: Be sure to leave in those #$&* marks in future assignments.