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: Dimensional analysis and frequency

  1. Aug 29, 2004 #1
    dimensional analysis (Please help as soon as possible!)

    The Space Shutte astronauts use a massing chair to measure their mass. The chair is attached to a spring and is free to oscillate back and forth. The frequency of the oscillation is measured and that is used to calculate the total mass m attached to the spring. If the spring constant k is measured in kg/s^2 and the chairs frequency f is .50s^-1 for a 62-kg astronaut, what is the chair's frequency for a 75-kg astronaut? The chair itself has a mass of 10.0 kg. [Hint: use dimensional analysis to find out how f depends on m and k.]

    Could you please explain how to work this problem step by step, because i have no clue how to even begin, thank you.
    Last edited: Aug 29, 2004
  2. jcsd
  3. Aug 29, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    We know that k is measured in kg/s^2, m is measured in kg, and f is measured in 1/s. I would write this as


    (M=mass, T=time)

    We want to find how f depends on k and m. We do this by solving the equation


    for a and b. Using what we know about the units, this equation takes the form


    The solution is obviously


    so we know that


    where A is a dimensionless constant.

    We can solve this equation for A^2k:


    Now you can calculate A^2k using the numbers f=0.5 and m=72. Then insert the result along with m=85 into the formula for f above.

    As you see, you don't need to know A and k separately. It's enough to know A^2k. If you would like to know what A is I can tell you that it's 1/(2pi), but you can't get that from dimensional analysis. You would have to solve the equation of motion (Newton's second law) to get that.
  4. Aug 29, 2004 #3
    Thanks for the help.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook