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Phys201
Your 'cq_1_05.1' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
** CQ_1_05.1_labelMessages **
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30 min
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A ball accelerates at 8 cm/s^2 for 3 seconds, starting with velocity 12 cm/s.
• What will be its velocity after the 3 seconds has elapsed?
answer/question/discussion: ->->->->->->->->->->->-> :
use change in velocity=12 cm/s-8cm/s^2=4 cm/s
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8 cm/s^2 has different units than 12 cm/s. So the two quantities are unlike terms, which cannot be added or subtracted.
Even more importantly, the two quantities measure different types of things. One is a velocity and the other is an acceleration. You can't add or subtract these any more than you could add or subtract dollars and donuts.
Note also the algebra of the calculation. We can factor out 4 cm/s:
12 cm/s - 8 cm/s^2 = 4 cm/s ( 3 - (2 / s) ).
3 - (2/s) is not something you can calculate or simplify.
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You can, however, calculate the change in velocity from the given information. Which two bits of information can be used to find the change in velocity?
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• Assuming that acceleration is constant, what will be its average velocity during this interval?
answer/question/discussion: ->->->->->->->->->->->-> :
vAvg=`ds/`dt
=4cm/s/3sec
=1.3cm/sec
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`ds / `dt does give you vAve.
`ds stands for a displacement.
4 cm/s is not a displacement. It doesn't have the units of displacement, and it wasn't calculated by a method that would result in a displacement.
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• How far will it travel during this interval?
answer/question/discussion: ->->->->->->->->->->->-> :
So this means you need to find displacement.
Displacement=velocity*time
1.3cm/sec*3sec=3.9 cm
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displacement = average velocity * time interval.
`ds = vAve * `dt, which is a direct algebraic rearrangement of the fact that vAve = `ds / `dt.
However 1.3 cm/s is not the average velocity for this situaiton.
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1.3 cm/s is not a reasonable result for average velocity in any case. The velocity starts at 12 cm/s and increases. It's never as low as 1.3 cm/s.
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You should consider making a sketch to represent this situation. Put an x on your paper for the starting point, put another x for where the object would be after one second if it moved at 12 cm/s. Then put two more x's to represent where it would be at the end of each of the next two seconds.
How far would the object have moved during each second, and how far during the 3 seconds?
Now put an o where you think the object would be if it started at 12 cm/s and accelerated at 8 cm/s^2 for a second. Ask yourself what its velocity would be at the end of this second, and how this would affect how far it traveled.
Then continue for the next two seconds.
It might also help to list the velocities at the ends of each of the first three seconds.
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