- #1
epidote6
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I'm more having a problem of what they actually want and what I can use/assume. The question is:
A horizontal disk of moment of inertia I rotates frictionlessly around its axis. A massless pulley of radius r which shares the axis of rotation is attached to the disk. A massless string is wound around the pulley, passed over another massless frictionless pulley, and attached to a hanging mass m.
There are two pictures, one of a rotating disk from above with Ft pointing out from it. And the second of a hanging mass from the side with Ft pointing up and mg pointing down from the mass.
a) write down equation of motion for rotating disk. Denote angular acceleration by alpha.
I just have F(tension) = I * alpha. I think that's all it is.
b) Write down the equation of motion for the hanging mass. Its acceleration a is related to ang acc. of disk by a = r*alpha.
Do I just plug in a/r for alpha?
C) Solve equations for a) and b) for Ft and alpha. (Express answers in terms of known quantities, namely, I, r, m, and g.)
This is the one I am confused about since for b) i don't have an alpha. Also isn't Ft = mg ? Maybe I am making this harder that it actually is. All my other problems were a lot harder for this to be as simple as I have it.
I appreciate the help.
A horizontal disk of moment of inertia I rotates frictionlessly around its axis. A massless pulley of radius r which shares the axis of rotation is attached to the disk. A massless string is wound around the pulley, passed over another massless frictionless pulley, and attached to a hanging mass m.
There are two pictures, one of a rotating disk from above with Ft pointing out from it. And the second of a hanging mass from the side with Ft pointing up and mg pointing down from the mass.
a) write down equation of motion for rotating disk. Denote angular acceleration by alpha.
I just have F(tension) = I * alpha. I think that's all it is.
b) Write down the equation of motion for the hanging mass. Its acceleration a is related to ang acc. of disk by a = r*alpha.
Do I just plug in a/r for alpha?
C) Solve equations for a) and b) for Ft and alpha. (Express answers in terms of known quantities, namely, I, r, m, and g.)
This is the one I am confused about since for b) i don't have an alpha. Also isn't Ft = mg ? Maybe I am making this harder that it actually is. All my other problems were a lot harder for this to be as simple as I have it.
I appreciate the help.