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Phy 121
Your 'cq_1_02.1' 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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The problem:
A ball starts with velocity 4 cm/sec and ends with a velocity of 10 cm/sec.
What is your best guess about the ball's average velocity?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
If a ball has a velocity of 4 cm/sec and has an end velocity of 10cm/sec than the average velocity would be somewhere between 4cm/sec and 10 cm/sec.
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That's right. If you don't know anything else about the motion your best guess would probably be 7 cm/s.
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Without further information, why is this just a guess?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
This is just a guess because we don't know any further information and are unable to
calculate any further. As to why the number is between 4cm/sec and 10cm/sec, it would be because 4 cm/sec was the beginning speed and thus the slowest and 10 cm/sec was the ending speed and hopefully the fastest that the ball would roll. Therefore the average speed would be somewhere between the two velocities.
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If it takes 3 seconds to get from the first velocity to the second, then what is your best guess about how far it traveled during that time?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
If it takes 3 seconds to get from the first veloctity to the second, then
the car is gaining 2 cm/sec every second. That means that the car would move 2 cm/sec*3 seconds, or 6 cm every second. Since the car travels for 3 seconds, the car will have moved 18 cm during the 3 seconds.
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You're right that the car gains 2 cm/s every second.
However that doesn't tell you how far it moves. An object could have speeds of 100 cm/s, 102 cm/s, 104 cm/s at the ends of three consecutive 1-second intervals. That object would move over 200 cm between the end of the first and the end of the third 1-second interval.
How quickly an object picks up speed doesn't by itself tell us how fast it's actually moving, or how far it moves.
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At what average rate did its velocity change with respect to clock time during this interval?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
The velocity of the car changed at an average rate of 2cm/sec during this interval?
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Good. You would say this as '2 cm/s per second'.
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Your instructor is trying to gauge the typical time spent by students on these questions. 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 much time did you spend on this question?
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I spent about 20 minutes on this question.
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I hope that I answered everything correctly.
#$&* self-critique
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These questions are designed so that a student is likely to answer some questions correctly, but also likely to need to think a little more about at least one thing.
You got just about everything, but be very sure to check my notes and let me know if you have questions.
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