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: Pendulum and elevator question

  1. Dec 31, 2007 #1
    1. The problem statement, all variables and given/known data
    A pendulum of length L is suspended from the ceiling of an elevator. When the elevator is at rest the period of the pendulum is T. How does the period of the pendulum change when the elevator moves upward with constant acceleration? Explain.

    2. Relevant equations

    T=2pi ([tex]\sqrt{l/g}[/tex])

    3. The attempt at a solution
    i know that..the period relies on the gravitational force which changes as the elevator is moving. i THINK that when the elevator moves up, gravity increases..but i am not sure why. please help!
  2. jcsd
  3. Dec 31, 2007 #2


    User Avatar
    Homework Helper

    If you are sitting in the bus and bus suddenly starts, you were thrown in the opposite direction. If you are sitting in the moving bus and if the driver suddenly applies the brakes you were thrown in the forward direction. Why is it so? Same reason applies to the lift problem.
  4. Dec 31, 2007 #3


    User Avatar
    Homework Helper

    I believe it is due to a thing called the "inertial force", it is an apparent force acting on the object when it is inside a non-inertial reference frame (such as the accelerating elevator)
    this is the prime reason why the "Foucault pendulum" can be used to test whether the Earth is rotating.
  5. Jan 2, 2008 #4

    Shooting Star

    User Avatar
    Homework Helper

    On all objects in the frame of reference of the lift, there acts the pseudo force or the inertial force -ma, opposite to the direction of a. It's as if there's an added g-field of strength a, acting downward, in addition to the actual g.

    Can you find T now?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook