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: Simple Physics (unit analysis)

  1. Sep 28, 2010 #1
    1. The problem statement, all variables and given/known data

    1. (5 points)
    If d is a distance, t is a time, and v is a speed, which of the following equations do unit analysis show must be wrong?

    A. d = vt
    B. v = 3 d^2 / t
    C. t = d/v
    D. v = d/t + 2 t/d
    E. v = d^2 / t^2


    I haven't done this since highschool and have no idea. It's pretty easy, but I don't remember this at all. Any help would be greatly appreciated, thanks guys!
     
  2. jcsd
  3. Sep 28, 2010 #2
    Here's an effort of what I think its asking for..

    A. d = vt
    KM = KM/s(s) KM =/= KM^2 [WRONG]

    B. v = 3 d^2 / t
    KM/s^2 = 3KM^2 / sec KM/s^2 =/= km^2/s [WRONG]

    C. t = d/v
    sec = KM / KM/s^2 sec =/= s^2 [WRONG]

    D. v = d/t + 2 t/d
    KM/s^2 = KM/s + 2s/KM ???

    E. v = d^2 / t^2
    KM/s^2 = KM^2 / s^2 ???


    idk.. >.<
     
  4. Sep 28, 2010 #3
    Velocity is in m/s not m/s^2 or km if you want though meters is the standard.
     
  5. Sep 28, 2010 #4

    fss

    User Avatar

    In addition to distance usually being in meters (m) and the unit for velocity being distance/time (m/s) as Chunkysalsa pointed out,

    For (D); do those two terms have the same units? If so, you're golden. If not, you have a problem.

    For (E) you've done the dimensional analysis correctly, so ask yourself if the right hand side is in units of distance/time.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook