# Homework Help: Rotational Motion Questions

1. Nov 14, 2005

### rockmorg

1 - A rotating door is made from four rectangular glass panes, as shown in the drawing. The mass of each pane is 95 kg. A person pushes on the outer edge of one pane with a force of F = 80 N that is directed perpendicular to the pane. Determine the magnitude of the door's angular acceleration.

This says it is about a fixed axis so I don't know how that changes it, but it doesnt seem like I have enough information for one of the rotational kinematics equations...

2 -

A thin uniform rod is initially positioned in the vertical direction, with its lower end attached to a frictionless axis that is mounted on the floor. The rod has a length of 1.90 m and is allowed to fall, starting from rest. Find the tangential speed of the free end of the rod, just before the rod hits the floor after rotating through 90°.

This one seems like it should be really easy, but I'm not sure. I'll need the time it takes for the rod to fall, and I assume that I could use moment of inertia somehow because they specifically say thin uniform rod? Then convert the angle it fell thru to radians...?

Thanks much, any help is appreciated!
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rockmorg

2. Nov 14, 2005

### Nm

For problem 1, first find the rotational inertia of the door (includes all 4 panes). Then, you can use the torque formula to find the angular acceleration. Remember the force is perpendicular to the pane.