# Hydrodynamics- rotational movement.

1. Mar 18, 2009

### bondmmo

Hydrodynamics--- rotational movement.

Hi,I am not sure whether my solution is correct. I am not an expert in this area either. Could anyone here help me to check my solution? Thank you

If i hang a block like a cube in water. If we look from the top view, there will be two side A and two side B, see the attached file . The object will be rotated only when current,drag forces, act on the side B. (Suppose the left and right side of side B area are not totally symmetrical and the moment can rotate the cube with very low speed because of small different area in Side B. Also, no moment can cause from side A)

When the object rotates, the force and moment will reduce. This results in non-constant angular acceleration, which I am not sure how to solve it. I calculated angular acceleration using Fxr=I$$\alpha$$.
However, I have done calculation in excel file using X-axis as $$\theta$$degree of rotation and Y-axis is force and moment. I tried to plot angular acceleration, velocity and time vs. the rotational degree. I set $$\theta$$ from 0 to 90 degree, with increase of 5 degrees.

Can i assume to use simply equation of motion to find them? are these following equations used for only a constant acceleration?

1. Can I useFxr=I$$\alpha$$ to find angular acceleration?
2. $$\omega$$^2=$$\omega$$ o ^2+ 2 $$\alpha$$ $$\theta$$ to find velocity?
3. $$\theta$$ =$$\theta$$o + $$\omega$$*t+ 0.5 *$$\alpha$$*t^2 . Can I solve this equation to find time when the object rotates from 0 to 90 degree?

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