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Lab Quiz 6

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Lab Quiz 6 Empty Lab Quiz 6

Post  hwilson Thu Oct 23, 2008 2:01 pm

Question 1: (1 point)

You spin a wheel to give it an initial angular acceleration. The angular velocity of the wheel decreases with time due to a frictional torque acting on the axle of the wheel. You plot the angular velocity as a function of time. When determining the slope of the linear graph, you measure a change of the angular velocity with a magnitude of 3.5 rad/s for a time interval of 11.4 s. What is the magnitude of the angular acceleration due to friction (see Ch8 sheet 5)? Indicate with positive (negative) sign whether the sign of the measured acceleration is negative (positive).

(How to solve):


Question 2: (1 point)



You calculate the moment of inertia of a disk with a handle attached as shown above. The mass of the disk + handle is 4.9 kg and the radius of the disk is 35.4 cm. You use the equation in Ch8 sheet 18, which holds for the moment of inertia of a uniform disk, and ignore the handle. What is the moment of inertia? Indicate with a negative (positive) sign whether you overestimate (underestimate) the moment of inertia of disk + handle, and the error you make is small (large). (Hint: when you judge whether the error made is small or large, consider the mass and the radius of the handle relative to the disk).

(How to solve):


Question 3: (1 point)



You measure the moment of inertia of a wheel by measuring its angular acceleration a = 0.413 rad/s2, when applying an external torque caused by a hanging weight of mass m = 200 g as shown above. The angular acceleration is decreased by a frictional torque, accounted for by afric = - 0.193 rad/s2 in the expression for the momemt of inertia I of the wheel, I = mr(g-ra) /(|afric|+a), where r is the radius of the cylinder attached to the wheel, and g is the acceleration of gravity, which you treat as error free. You neglect the error of the accelerations a and afric, and the error of the mass m. You find out that the term (a r) in the numerator is small compared to the term g, and thus neglect its error too. What is the absolute error of I, if the 2.5 cm radius r is known with a 1.3 % accuracy. (Hint: Read in the manual of the lab, how I can be written as two factors. One factor contains the quantities assumed to be error free, after taking into account that the term (a r) can be neglected for the purpose of error calculation. Use expres​sion(3) and (4) in "Error and Uncertainty".) Indicate with a positive (negative) sign whether, with the neglections you made above, the absolute (relative) errors of I and r are the same.


(How to solve):

hwilson
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Lab Quiz 6 Empty Re: Lab Quiz 6

Post  Andrew Sat Oct 25, 2008 1:59 pm

Question 1:

Answer is positive

angular velocity / time

Question 2:

Answer is negative

.5 (constant for a disc) * mass * radius(in meters)^2

Any ideas for Question 3?

Andrew
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Lab Quiz 6 Empty question 3

Post  chen Sat Oct 25, 2008 8:22 pm

answer: negative

after you done all error neglect and calculate out the I

the error equation should be
DeltaI(absolute error of I)=sqrt((Deltar/r)^2)*I
DeltaI=Deltar/r*I I is calulated through m*r*(g-ar)/(afric+a), where afric is absolute vaule, ar=angular accelerration * radius

the answer is negative because DeltaI*I=Deltar*r

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Post  chen Sat Oct 25, 2008 9:17 pm

The answer works on the CD practice question, but it is not working on maple TA, i don't know why, i try many times already, still not corrected.
does anyone have any idea?

chen
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Post  chen Sat Oct 25, 2008 9:18 pm

Deltar/r=percentage given in the question

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Lab Quiz 6 Empty q3

Post  help me Sun Oct 26, 2008 8:48 pm

is the mass supposed it be in kg?

help me
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Lab Quiz 6 Empty question 3

Post  hwilson Sun Oct 26, 2008 10:33 pm

I keep getting question three wrong could someone please post a step by step procedure for it?

hwilson
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Lab Quiz 6 Empty has anyone gotten #3 correct?

Post  phyguest Mon Oct 27, 2008 10:25 pm

Ive tried question 3 using

deltar/r = deltaI/I

as well as just deltar/r and neither came out right... (i put answers in as negative)

does anyone know how to do this problem/has anyone gotten it correct on mapleTA? thanks! Smile

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Post  Guest01 Mon Oct 27, 2008 11:18 pm

I am still having trouble with Question 3 as well.

Guest01

Posts : 133
Join date : 2008-09-19

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Lab Quiz 6 Empty ques 3

Post  K Fri Nov 07, 2008 1:34 am

any insights on # 3 yet?

K
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Lab Quiz 6 Empty Re: Lab Quiz 6

Post  Question Fri Nov 07, 2008 11:20 am

can someone who had #3 correct post exactly what they did because the one posted above is not understandable..thanks

Question
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Post  waiting Fri Nov 07, 2008 4:46 pm

it says to refer to the lab procedure but it is not ready yet (on black board).

waiting
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Lab Quiz 6 Empty question3

Post  stranger Fri Nov 07, 2008 9:36 pm

question3
multiply the error of r they give(over 100) by I, which you work out using the formula given

stranger
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Lab Quiz 6 Empty ques 3

Post  confused Sat Nov 08, 2008 5:40 pm

Number 3 still wont work for me, can you help me figure out what i did wrong?

angular acc = .388
afric = -.171
r=.025 m
m= .2kg
error of r = 1.35%

1.35/100 = .0135

I = mr (g-r*angularacc) / (abs afric + angular acc)
I= (.2)(.025) * (9.81-(.025*.388)) / (.171 + .388)
= .087659

.087659 * .0135 = .0011833

and i put it as negative. any suggestions?

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Post  k Sat Nov 08, 2008 5:45 pm

ive tried it with both grams and kg so idk which is right, and are we sure its negative?

k
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Post  guesto Sat Nov 08, 2008 5:50 pm

Strangers worked for me. Symbols they use for the variables in the question is confusing tho...

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Post  guesto Sat Nov 08, 2008 5:53 pm

and in reply to confused, maybe you had rounded off too early because the answer I got at the end without rounding off before hand is .001182

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Post  Guest01 Sat Nov 08, 2008 10:12 pm

It's still not working for me either

Guest01

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Lab Quiz 6 Empty answer to question 3

Post  tofu Sun Nov 09, 2008 3:16 am

The following steps worked out for me. This is what I did:

Wanted: Delta I= I * Delta r/ r
First find I=mg/(alpha fric+alpha) Note: take the absolute value of alpha friction.
Then, find Deta r/r = (% accuracy/100 * radius
Finally, multiply two values together.
(it's negative)

Hope it helps and will work for you guys!

tofu
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Lab Quiz 6 Empty help

Post  student Sun Nov 09, 2008 2:24 pm

Could you help me with number 3?

student
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Lab Quiz 6 Empty thanks

Post  student Sun Nov 09, 2008 2:33 pm

thank you, it's work for number 3 Very Happy

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Post  Kathleen Sun Nov 09, 2008 2:59 pm

Yeah, tofu's suggestion for question 3 works, and remember to put the mass in kg!

Kathleen
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Post  abc Sun Nov 09, 2008 7:08 pm

I am still having trouble with Question 3

can someone plug in the numbers so we get it

abc
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Post  help!!! Mon Nov 10, 2008 12:44 am

Tofu your solution worked!!!!!! THANKS

help!!!
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Post  Kathleen Mon Nov 10, 2008 12:58 am

You measure the moment of inertia of a wheel by measuring its angular acceleration a = 0.413 rad/s2, when applying an external torque caused by a hanging weight of mass m = 200 g as shown above. The angular acceleration is decreased by a frictional torque, accounted for by afric = - 0.193 rad/s2 in the expression for the momemt of inertia I of the wheel, I = mr(g-ra) /(|afric|+a), where r is the radius of the cylinder attached to the wheel, and g is the acceleration of gravity, which you treat as error free. You neglect the error of the accelerations a and afric, and the error of the mass m. You find out that the term (a r) in the numerator is small compared to the term g, and thus neglect its error too. What is the absolute error of I, if the 2.5 cm radius r is known with a 1.3 % accuracy. (Hint: Read in the manual of the lab, how I can be written as two factors. One factor contains the quantities assumed to be error free, after taking into account that the term (a r) can be neglected for the purpose of error calculation. Use expres​sion(3) and (4) in "Error and Uncertainty".) Indicate with a positive (negative) sign whether, with the neglections you made above, the absolute (relative) errors of I and r are the same.

Okay for those that wanted it step by step, using tofus method this is how to do it

delta r/r = %/100 * radius (in meters) so you do (1.3/100) * .025 m = .000325

put mass into kg for this next part, and use the positive afric since it asks for absolute value
I = mg/ (|afric|+angacceleration) = (.2 g * 9.81) / (.193 + .413) = (1.962)/(.606) = 3.2376

(.000325)*(3.2376) = answer = .00105222

And it is negative

hope that helps

Kathleen
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