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06-13-2012, 01:15 AM   #31
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Re: Mechanics exam question is impossible?

Quote:
 Originally Posted by dadude77 ok I apologize, I translated the problem from french and it wasn't perfect translation. The actual problem says that the Force of friction, mass and radius of all three are the SAME. The same values are to be used for both cylinders and the sphere.
In that case the hollow cylinder will accelerate more slowly than the two solid objects (which will tie).

See just after the 10:00 mark here for the formulas and demo:

Last edited by NewOldGuy; 06-13-2012 at 01:32 AM.

06-13-2012, 10:12 AM   #32
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Re: Mechanics exam question is impossible?

Quote:
 Originally Posted by NewOldGuy In that case the hollow cylinder will accelerate more slowly than the two solid objects (which will tie). See just after the 10:00 mark here for the formulas and demo:
That is not true. The sphere will be faster. The sphere has more mass concentrated at the axis of rotation than the cylinder and therefore a lower moment of inertia.

You can see it from the equations for I given in the problem statement.

I=.5mrČ for full cylinder
I=mrČ for hollow cylinder
I=.4mrČ for full sphyere

mr^2 is the same for all three so the term in front contains all of the information needed to order their arrival at the bottom. Lowest first to highest last.

06-17-2012, 09:41 PM   #33
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Re: Mechanics exam question is impossible?

Quote:
 Originally Posted by dadude77 This is also why above someone said it is "torque" not the moment. In french torque is called the "moment" and moment of inertia is different
FYI, in English we call force at a distance a moment as well (at least in engineering). Also just as an FYI in structural engineering we retardedly use moment of inertia to refer to the section's second moment of area/resistance to bending.

06-18-2012, 04:59 PM   #34
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Re: Mechanics exam question is impossible?

Quote:
 Originally Posted by RLK That is not true. The sphere will be faster.
I stand corrected.

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