1. Limited time only! Sign up for a free 30min personal 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!

Linear velocity!

  1. May 24, 2014 #1
    1. The problem statement, all variables and given/known data

    What is the linear velocity of the foot if the angular velocity of the dynamometer was 180 degrees/s and the distance from the knee joint to the foot is 0.5 m. Give your answer rounded to two decimal places in m.s-1. (Do not include units in answer).

    2. Relevant equations

    linear velocity = angular velocity (w) x radius (r).
    linear velocity = w x r.

    3. The attempt at a solution

    w = 180 degree/s
    r = 0.5m
    linear velocity = ?

    linear velocity = w x r
    linear velocity = 180 x 0.5
    linear velocity = 90 degrees/s

    To find radians, 90/57.3 = 1.57 m/s

    Is this correct? And in the right units?

    Please help and thank you! :)
     
  2. jcsd
  3. May 24, 2014 #2

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Well, your relevant equation gives ##0.5 {\rm m}\, \times \,\pi \, {\rm rad/s} = 1.57 \,{\rm m/s}## straightforward.

    But your dealing with units is a bit messy. Especially the 90 degrees/s is objectionable. A linear velocity has dimension length/time. The numerics are fine, but the dimensions should read something like: degrees/s / radians/degree x meters/radian

    THe writer of the exercise doesn't want to be bothered by these considerations, so he/she asks for a number only.

    In my first sentence I let the radians disappear, you could say that I cheat too. My defence is: radians is arc per radius, so length/length and thereby dimensionless: it can be skipped in dimensional expressions.
     
  4. May 24, 2014 #3
    Thank you so much! Great help :)
     
  5. May 24, 2014 #4
    It is easier to understand if you convert to radians first and then calculate the linear velocity.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Linear velocity!
  1. Linear velocity (Replies: 1)

Loading...