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course Phy 201
10/19 1:30 pmLab-related Questions for 101013
Note: Before doing the lab questions you should run through the Sketching Exercise below. That exercise starts with questions about masses pulled
upward by tension and downward by gravity, much along the lines discussed in class. It continues with questions related to masses on inclines.
In 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)?
*****Length is 11 cm. T=.2sqrt (11 cm)=.66
`qx002. Give the time from release to first, second, third and fourth 'strikes' of the pendulum.
*****The time from release to first, second, third, and fourth strikes is a 4 count out of 8. Each count represents .18s. so 4*.18=.72 s
`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.
*****
13, 13, 13, 14
13.25
2.39 s
17.5cm/s^2
13.25*.18s=2.39 s; Vave=50 cm/2.39s=20.9cm/s Vf=41.8cm/s A=41.8 cm/s/2.39s=17.5 cm/s^2
`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.
****
4,4,5,4
4.25
.77 s
168.7 cm/s^2
`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.
****
3,3,3,3
3
.54 s
342.9 cm/s^2
Your pendulum strikes would have been separated in time by half of the .66 second period, or .33 seconds. This would affect your results. However your reasoning process was otherwise correct throughout.
`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.
`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?
****
Vector showing gravitational force was greater by about 40 %, the Fnet is about 35% of gravitational force.
`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?
****
The tension vector was longer by about 45%, the Fnet on these clips are about 60 % of gravitational force.
the acceleration you got was abou 340 cm/s^2, which is about 35% of the acceleration of gravity. So the net force vector should be 35% as long as the gravitational force vector.
`q009. At what average rate does the acceleration of the system change with respect to the number of small paperclips?
****With each additional small paperclip, the acceleration increases by about 10 times that of the original acceleration.
the original acceleration isn't a particularly significant quantity, so we probably wouldn't use it as a basis for comparison
in any case you got numerical values for the accelerations, in cm/s^2, so you can give an answer in terms of your results
`q010. How much acceleration do we tend to be gaining, per added paperclip?
****We gain approximately 10 times the amount of the original acceleration per each additional paperclip.
`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?
****We get about 15% of the acceleration of gravity from each small clip. A single clip weighs approx 1 gram and each big clip about 10 grams. Can you
assume that adding the single clip is like adding 15% of the systems weight? Im not sure about this one. Its a 10:1 ratio.
that's right, according to your calculations, and your statement pretty much answers a couple of the preceding questions
If the single clip was 15% of the total mass, then what proportion of the mass of a large clip would this imply?
Your accelerations are greater than they would be if you used the .33 second time between 'strikes' of the pendulum, so if you recalculated the accelerations that 15% would be reduced.
`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).
&#Good responses. Let me know if you have questions. &#
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