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course Phy 241
October 25 around 6:30pm.
Lab-related Questions for 101013In lab you timed the Atwood machine (paperclips on pulley) using your bracket pendulum.
`qx001. What was the length of your pendulum? What would be the period of a pendulum of this length, based on T = .2 sqrt(L)?
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11cm
Period=0.663
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`qx002. Give the time from release to first, second, third and fourth 'strikes' of the pendulum.
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7 counts (0.17sec*7counts=1.2 seconds)
(1 of my personal counts are 0.17 seconds)
if you are using the pendulum to pace your counts, then the intervals would be dictated by the period of the pendulum
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`qx003. In your first set of trials there were 3 large clips on each side.
• In the first line give your counts for the first set of trials, separated by commas.
• In the second line give the mean of your counts.
• In the third line give the time interval in seconds which is equivalent to the mean of your counts.
• In the fourth line give the acceleration corresponding to the time interval just reported.
• Starting in the fifth line give an explanation of the results you gave in the third and fourth lines.
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8-count 2, 8-count 5, 8-count 4, 8-count 4, 8-count 5
12 counts (8-count 4)
2.04 seconds (0.17seconds*12counts)
24.03cm/s^2
I got the acceleration by using the formula (‘ds/0.5’dt^2=a) and solved for “a.” I used 50cm for ‘ds and used 2.04 seconds for ‘dt.
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`qx004. In your second set of trials there were still 3 large clips on each side, but there was a small clip on the side which ascended in the first set.
• In the first line give your counts for this set of trials, separated by commas.
• In the second line give the mean of your counts.
• In the third line give the time interval in seconds which is equivalent to the mean of your counts.
• In the fourth line give the acceleration corresponding to the time interval just reported.
• You don't need to include an explanation, since the procedure is identical to that of the preceding questions, which you explained in answering that question. Just make sure your results make sense.
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4, 5, 5, 4
4.5 counts
0.765 seconds (0.17seconds*4.5counts)
170.65cm/s^2
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`qx005. In the third set of trials a second small clip was added to each side.
• In the first line give your counts for this set of trials, separated by commas.
• In the second line give the mean of your counts.
• In the third line give the time interval in seconds which is equivalent to the mean of your counts.
• In the fourth line give the acceleration corresponding to the time interval just reported.
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3, 3, 3, 3
3 counts
0.51seconds (0.17seconds*3counts)
384.47cm/s^2
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`qx006. If there was a fourth set of trials, report as before:
• In the first line give your counts for this set of trials, separated by commas.
• In the second line give the mean of your counts.
• In the third line give the time interval in seconds which is equivalent to the mean of your counts.
• In the fourth line give the acceleration corresponding to the time interval just reported.
NO FORTH SET.
`qx007. For the trial with the greatest acceleration, sketch a force diagram showing, to scale, the tension and gravitational forces acting on the clips on the descending side of the system.
• Which vector was longer?
• By what percent was it longer?
• What is the net force on these clips as a percent of the gravitational force?
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Mg vector
60% longer
Fnet=(m_descending*g)-T (40%)
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`qx008. For the trial with the greatest acceleration, sketch a force diagram showing, to scale, the tension and gravitational forces acting on the clips on the ascending side of the system.
• Which vector was longer?
• By what percent was it longer?
• What is the net force on these clips as a percent of the gravitational force?
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Tension vector
60% longer
Fnet=T-(m_ascending*g) (40%)
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`q009. At what average rate does the acceleration of the system change with respect to the number of small paperclips?
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From 6 large clips to an added small clip, the accelerations changed by about 150cm/s^2.
From the first added small clip to another small clip, the accelerations changed by about 200cm/s^2.
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`q010. How much acceleration do we tend to be gaining, per added paperclip?
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Approx. 180cm/s^2
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`q011. The unbalance in the gravitational forces with each new paperclip is of course significant. It is this unbalance that causes the differences in the system's acceleration.
The total mass of the system does increase slightly with each added small paperclip, but for the moment let's assume that the resulting change in the total mass of the system isn't significant.
• What percent of the acceleration of gravity do we get from each added small clip?
• How is this related to the mass of a single clip as a percent of the system's total mass?
• What is your conclusion about the ratio of the mass of a large clip to the mass of a small clip?
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15%
1.5%
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`q012. This question can be challenging. Don't let yourself get too bogged down on it:
In the preceding you drew conclusions based on the assumption that the changes in the system's total mass due to adding up to a few small paperclips was insignificant. It is perfectly possible that uncertainties in measuring the time intervals were large enough to obscure the effect of the changes in the total mass.
However refine your answers to the preceding question to take account of the change in total system mass.
(One possible approach: assume that the requested ratio is r and symbolically solve for the acceleration a in terms of the number N of added small clips, sketch a graph showing the predicted shape of your a vs. N curve, and see what value of r best matches this graph with a graph of your observed a vs. N).
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Good. Your time intervals probably differed by about double the .17 seconds you used, so your accelerations would have been about 1/4 as great. This would affect most of your answers, but your answers followed correctly from the accelerations you did calculate, so there's no need to redo anything.
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