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!

Pendulum Problem

  1. Jan 14, 2008 #1

    bfr

    User Avatar

    1. The problem statement, all variables and given/known data

    The bob of a pendulum 1.2m long is pulled aside so the string is 40 degrees from the vertical. When the bob is released, with what speed will it pass through the bottom of its path?

    2. Relevant equations

    PE=mgh
    KE=(mv^2)/2

    3. The attempt at a solution

    Well, I started out with cos 40=x/1.2 and found x to be approximately .92, but I don't exactly know where to go from there.
     
  2. jcsd
  3. Jan 14, 2008 #2

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Well, you wrote down the expressions for KE and PE. What do you think you should do with them? Think conservation.
     
  4. Jan 14, 2008 #3

    bfr

    User Avatar

    Wait...PE+KE is always a constant, right? So when the bob is first released at 40 degrees, it as zero kinetic energy, and PE=mgh=9.8(1.2-.92)~=2.75, where .92 is the solution to "cos 40=x/1.2". So, at the bottom of its path, it's height will be zero...right? Which leaves me with m(9.8)(0)+.5(m)(v^2)=2.75. Can I just eliminate "m" from the equation and solve from there?
     
  5. Jan 14, 2008 #4

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You're close.

    There should be an m on the right hand side. You've only accounted for the gh.

    You can eliminate the m, but only after you make the correction on the right side of the equation. Do you see what I'm talking about?
     
  6. Jan 14, 2008 #5

    bfr

    User Avatar

    Oh, yeah...thanks!

    So m(9.8)(0)+.5(m)(v^2)=2.75m -> .5v^2=2.75 -> v~=2.35 ?
     
  7. Jan 15, 2008 #6

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes, that's it.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Pendulum Problem
  1. Pendulum problem (Replies: 3)

  2. Pendulum Problem (Replies: 6)

  3. Pendulum Problem (Replies: 3)

  4. Pendulum Problems (Replies: 2)

Loading...