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Homework Help: What is the rotational inertia of the door about the hinges?

  1. Apr 13, 2008 #1
    Thank you all for taking the time to help me out. Here's what I'm given:

    1. The problem statement, all variables and given/known data
    A solid door of mass M = 19 kg and width = 128 cm is hit at a right angle by a mud ball of mass m = 0.6 kg, which, as Fig. 10–44 shows, hits the door at the edge with speed v = 16 m/s and sticks.

    Figure 10-44

    (a) What is the rotational inertia of the door about the hinges?
    10.377 kg·m2
    (b) What is the angular velocity of the door after having been struck?
    ________ rad/s
    (c) What fraction of the initial energy does the moving door–mud ball system retain?

    2. Relevant equations
    I= 1/3mr^2

    3. The attempt at a solution
    -Part A was simply 1/3mr^2
    -Part B confuses me because I have to somehow make a linear momentum into a rotational momentum, while not forgetting the moments of inertia. Someone please shed some insight to this simple problem.
    -Part C = I need part B :/
  2. jcsd
  3. Apr 13, 2008 #2


    User Avatar
    Science Advisor

    Your equation L=mvr is what you need for part B. This will give you the angular momentum of the projectile.
  4. Apr 13, 2008 #3
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