If your solution to stated problem does not match the given solution, you should self-critique per instructions at
http://vhcc2.vhcc.edu/dsmith/geninfo/labrynth_created_fall_05/levl1_22/levl2_81/file3_259.htm.
Your solution, attempt at solution.
If you are unable to attempt a solution, give a phrase-by-phrase interpretation of the problem along with a statement of what you do or do not understand about it. This response should be given, based on the work you did in completing the assignment, before you look at the given solution.
030. Rotational Motion
Question: `q001. Note that this assignment contains 5 questions.
If an object rotates through an angle of 20 degrees in five seconds, then at what rate is angle changing?
Your solution:
Confidence rating:
Given Solution:
The change of 20 degrees in 5 seconds implies a rate of change of 20 degrees / (5 seconds) = 4 deg / sec. We call this the angular velocity of the object, and we designate angular velocity by the symbol `omega.
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Question: `q002. What is the average angular velocity of an object which rotates through an angle of 10 `pi radians in 2 seconds?
Your solution:
Confidence rating:
Given Solution:
The average angular velocity is equal to the angular displacement divided by the time required for that displacement, in this case giving us
`omega = `d`theta / `dt = 10 `pi radians / 2 seconds = 5 `pi rad/s.
STUDENT QUESTION
I write 5’pi radians as 15.7 radians. I
know they equal each other, but would you rather see me write it as 5’pi??
INSTRUCTOR RESPONSE
5 pi is exact and 15.7 is not. The rounding error in the approximation 15.7 might or might not be significant in a given situation.
Also it's easy to see how 5 pi is related
to the conditions of the problem; 15.7 is not as obviously related.
So in this case the multiple-of-pi notation is preferable, though either would
be acceptable.
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Question: `q003. If an object begins with an angular velocity of 3 radians / sec and ends up 10 seconds later within angular velocity of 8 radians / sec, and if the angular velocity changes at a constant rate, then what is the average angular velocity of the object? In this case through how many radians this the object rotate and at what average rate does the angular velocity change?
Your solution:
Confidence rating:
Given Solution:
Starting at 3 rad/s and ending up at 8 rad/s, the average angular velocity would be expected to be greater than the minimum 3 rad/s and less than the maximum 8 rad/s. If the angular velocity changes at a constant rate, we would in fact expect the average angular velocity to lie halfway between 3 rad/s and 8 rad/s, at the average value (8 rad/s + 3 rad/s) / 2 = 5.5 rad/s.
Moving at this average angular velocity for 10 sec the object would rotate through 5.5 rad/s * 10 s = 55 rad in 10 sec.
The change in the angular velocity during this 10 seconds is (8 rad/s - 3 rad/s) = 5 rad/s; this change takes place in 10 seconds so that the average rate at which the angular velocity changes must be ( 5 rad / sec ) / (10 sec) = .5 rad/s^2. This is called the average angular acceleration.
Angular acceleration is designated by the symbol lpha. Since the angular velocity in this example changes at a constant rate, the angular acceleration is constant and we therefore say that
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Given Solution:
`alpha = `d `omega / `dt.
Again in this case `d`omega is the 5 rad/sec change in the angular velocity.
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Question: `q004. If an object starts out with angular velocity 14 rad/s and accelerates at a rate of 4 rad/s^2 for 5 seconds, then at what rate is the object rotating after the 5 seconds? Through how many radians will the object rotate during this time?
Your solution:
Confidence rating:
Given Solution:
Changing angular velocity at the rate of 4 rad/s^2 for 5 sec the angular velocity will change by (4 rad/s^2) (5s) = 20 rad/s.
Since the angular velocity was already 14 rad/s at the beginning of this time period, it will be 14 rad/s + 20 rad/s = 34 rad/s at the end of the time period.
The uniform rate of change of angular velocity implies that the average angular velocity is (14 rad/s + 34 rad/s) / 2 = 24 rad/s.
An average angular velocity of 24 radians/second, in 5 seconds the object will rotate through an angle `d`theta = (24 rad/s) ( 5 sec) = 120 rad.
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If you understand the assignment and were able to solve the previously given problems from your worksheets, you should be able to complete most of the following problems quickly and easily. If you experience difficulty with some of these problems, you will be given notes and we will work to resolve difficulties.
Question: `q005. A rotating object starts with an angular velocity of 12 radians / second and accelerates through 60 radians in 6 seconds. What are its final angular velocity and angular acceleration?
Your Solution:
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