Lab Quiz 6

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 expression (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):

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?

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

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?

Deltar/r=percentage given in the question

is the mass supposed it be in kg?

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

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!

I am still having trouble with Question 3 as well.

any insights on # 3 yet?

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

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

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

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?

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