initial timing experiment

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Phy 121

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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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 our physics book ISBN 0-13-060620-0 which I measured to be 28.3 cm long. I stacked 3 quarters under the book to create an incline. I used a full can of V8 juice 5.5 fl oz. (163 ml) as my moving object.

I set the book on the table with two stacks of 3 quarters on one side of the book to create a ramp. I had another book Statistics ISBN ISBN 0-13-187706-2 with a piece of styrofoam in front of it as my stopping point.

My starting point for the an was 3.4 cm at the top of the ramp therefore it only rolled 24.9 cm.

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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:

During the first part of the experiment, the first time I measured the time it took the can of V8 juice to roll down the book, it took the can of V8 juice 2.625 seconds to roll 24.9 cm down the book. The second time it took 2.7539 seconds, the third 2.7226 seconds.

Then I put the quarters under the book on the opposite end, the first time it took the can 4.22265 seconds to reach the styrofoam. The second time it took 4.433594 seconds to roll down the book, and 4.390625 seconds the third time.

During each experimental run the can was released from the top of the ramp and rolled 24.9 cm down the book. Once the can was released the timer was started, and once the styrofoam was reached the timer was stopped.

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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:

For the first 3 runs the average time was 2.7005 seconds for the can to reach the Styrofoam. This meant on average it took the can 0.10845 seconds to travel one cm.

During the second 3 runs the average time was 4.34896 seconds for the can to reach the Styrofoam. This meant on average it took the can 0.174657 seconds to travel one cm.

The measured book length minus the starting point of the can equaled 24.9 cm, therefore by taking the average of time and dividing it by the total distance traveled gives me the average speed per cm.

I think the time is accurate within .01 seconds, due to human triggerring.

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That calculation would give you the time required to move a centimeter, which would be less if the speed was greater.

So clearly this cannot be the speed.

Right idea, though. Just divide in the opposite order.

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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:

For the first 3 runs the initial speed was 0 cm per second. The average speed was 9.22084 cm per second. I think the can was moving at a greater speed then average speed when it reached the lower end of the book.

As the can goes down a steady incline, it will continue to increase in speed due to gravtional pull.

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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:

intial speed, average speed:change of speed, final speed.

Inital speed is 0. Average speed is the total time divided by distance. Change in speed is a the same as average speed, because you have to take intervals and time to get an average number.

Final speed would be the final minus intial which would be the greatest of speed.

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If an object starts at 0 cm/s and ends up at 10 cm/s, then what would be its average speed?

By how much did its speed change?

The two results are clearly different.

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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:

I added a 10cm and 20 cm point along the book. I took 3 timed measurements at 10 cm, and 3 timed measurements at 20cm as the can rolled down the book.

At 10cm I got 3 runs of .9430 s, .9355 s, .9345 s.

At 20cm I got 3 runs of 1.832 s, 1.848 s, 1.849 s.

So at 10 cm it took on average .0938 seconds to reach 1 cm.

From 10 cm to 20 cm on average it took .0905 seconds to reach 1 cm, this shows the can is moving faster as it goes down the ramp.

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Your average speeds would be about 10.6 cm/s for the 10 cm trials, and about 10.7 cm/s for the 20 cm trials.

This is a pretty small difference. If the ball was rolling down an incline, we would expectd a greater difference in average speed. It's not clear that the difference in these average speeds is significant.

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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 hour 15 mins

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You may also include optional comments and/or questions.

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Check my notes and let me know if you have any questions.

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