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course Phy 231
2-3-11 about 12:00am
The problem:A graph is constructed representing velocity vs. clock time for the interval between clock times t = 5 seconds and t = 13 seconds. The graph consists of a straight line from the point (5 sec, 16 cm/s) to the point (13 sec, 40 cm/s).
• What is the clock time at the midpoint of this interval?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
9sec
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• What is the velocity at the midpoint of this interval?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
(40cm/sec - 16cm/sec)/8sec= (3cm/sec)/sec
(3cm/sec)/sec * 4 sec= 12cm/sec
16cm/sec + 12cm/sec= 28cm/sec= velocity at the mid point
@& Good.
You could also find this by averaging 16 cm/s and 40 cm/s.*@
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• How far do you think the object travels during this interval?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
&&&&
ds= 28cm/sec * 4sec= 112cm
&&&&
@& You need to explain this.
Your answer is clearly incorrect. The object travels for 8 seconds and its velocity is never less than 16 cm/s, not consistent with a 12 cm displacement.*@
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• By how much does the clock time change during this interval?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
4 sec
@& This isn't correct, and it hasn't been explained.*@
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• By how much does velocity change during this interval?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
12cm/sec
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• What is the average rate of change of velocity with respect to clock time on this interval?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
3cm/sec/sec
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• What is the rise of the graph between these points?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
24cm/sec
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• What is the run of the graph between these points?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
8sec
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• What is the slope of the graph between these points?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
m=3
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• What does the slope of the graph tell you about the motion of the object during this interval?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
its moving at 3cm/sec/sec which is the acceleration
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• What is the average rate of change of the object's velocity with respect to clock time during this interval?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
3cm/sec/sec
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** **
@& You have a number of correct answers and a number of answers which are at least partially incorrect. However you need to explain all your reasoning.
You did so on the first question, but not on any of the others.*@
@& There is at least one response that is still incorrect, and which hasn't been explained.
You need to insert at least brief explanations for all answers.
I don't need to see full grammatical sentences, so you can abbreviate quite a bit, but I need to know what you're thinking.
*@