1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Angular momentum (left or right movement)

  1. Mar 30, 2012 #1
    1. The problem statement, all variables and given/known data

    A thin uniform rectangular sign hangs vertically above the
    door of a shop. The sign is hinged to a stationary horizontal
    rod along its top edge. The mass of the sign is 2.40 kg
    and its vertical dimension is 50.0 cm. The sign is swinging
    without friction, becoming a tempting target for children
    armed with snowballs. The maximum angular displacement
    of the sign is 25.0° on both sides of the vertical. At a
    moment when the sign is vertical and moving to the left, a
    snowball of mass 400 g, traveling horizontally with a velocity
    of 160 cm/s to the right, strikes perpendicularly the
    lower edge of the sign and sticks there.

    Is the system (sign-snowball) going to move left or right?

    2. Relevant equations

    Conservation of angular momentum.

    3. The attempt at a solution

    Lost
     
  2. jcsd
  3. Mar 30, 2012 #2

    Doc Al

    User Avatar

    Staff: Mentor

    What's the angular momentum of the sign when it passes the vertical? Hint: Start by finding its moment of inertia.
     
  4. Mar 31, 2012 #3
    Thank you for your reply.

    The moment of inertia is [itex]I=\frac{1}{3}ML^2[/itex], therefore the angular momentum of the sign when it passes the vertical is [itex]L_s=Iω=(\frac{1}{3}ML^2)ω[/itex].
     
  5. Mar 31, 2012 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Excellent. Now find ω.

    Once you've done that, figure out the initial angular momentum of the snowball.
     
  6. Apr 1, 2012 #5
    I found the angular velocity ω from the conservation of energy:

    [itex]\frac{1}{2}Iω^2=Mgh[/itex]
    [itex]ω=\sqrt{\frac{3g(1-cosθ)}{L}}=2.35 rad/s[/itex]

    The angular momentum of the sign is [itex]L_s=\frac{1}{3}ML^2=0.47 Nms[/itex] while the angular momentum of the snowball is [itex]L_b=mvL=0.32 Nms[/itex]

    Comparing these two, the angular momentum of the sign is greater than the angular momentum of the ball, therefore the system is going to move to the left.
     
  7. Apr 1, 2012 #6

    Doc Al

    User Avatar

    Staff: Mentor

    Good work!
     
  8. Apr 1, 2012 #7
    Thank you for your help and for checking my work. I really appreciate it.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Angular momentum (left or right movement)
  1. Force Left and Right (Replies: 1)

Loading...