Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Rotational Motion Questions

  1. Nov 14, 2005 #1
    Hey all, thought I should ask about a couple problems...

    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!
  2. jcsd
  3. Nov 14, 2005 #2


    User Avatar

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook