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Homework Help: Linear acceleration and moments of intertia

  1. Jul 21, 2008 #1
    Im having problems with two questions, the first is linear acceleration, A 6.00-kg cylindrical reel with a radius of 0.600 m and a frictionless axle, starts from rest and speeds up uniformly as a 4.00-kg bucket falls into a well, making a light rope unwind from the reel. The bucket starts from rest and falls for 4.00 s. Find the linear acceleration of the falling bucket.

    The formula I am using which I am not sure if it is correct is tmg/mt(1/2)M where t is time, m is the mass of the bucket, g is gravity, and M is the mass of the reel.

    attempting to solve this problem I always end up with 3.26, somewhere I think I have my math or variables mixed up and cant come to the right conclusion

    the second problem I am having trouble with is moments of interia, Four objects are held in position at the corners of a rectangle by light rods with M1=2.70kg and located at (-2m,3m), M2=1.70kg and located at (2m,3m), M3=4.10kg and located at (2m,-3m) and M4=2.10kg and located at (-2m,-3m). M stands for the masses of the objects. Find the moments of intertia along the x axis and the y axis respectively.

    I found the moment of intertia of the system at the origin of (0m,0m) by adding the x and y coordinates and squaring them to get 9 for the distance of each mass and then multiplying that by each mass and adding each individual mass together to get the answer of 137.8kg/m^2 however that formula doesnt seem to work when I try to find the x axis or y axis moment of intertia by multiplying the radius of each mass by its respective mass then adding all 4 masses together. please help.
  2. jcsd
  3. Jul 22, 2008 #2


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    Homework Helper

    Hi yugo2113,

    This formula is not correct. If you notice, it doesn't have the right units (m/(kg s^2) ).

    However, the formula is very close to being right. After the time cancels (which is good since the acceleration does not depend on time), it looks to me like you might have just made a simple algebra error. Try looking over your work again, and if you can't find the error please post your work.

    I don't think I follow what you've done. If you're trying to find the moment of inertia about the x-axis, you need the distance r that each mass is from the x-axis. What is that distance for each mass? Do you get the right answer for the moment of inertia?
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