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!

Homework Help: What are my units when finding % diff. In kinetic energy?

  1. Oct 4, 2013 #1

    astru025

    User Avatar

    What are my units when finding % diff. In kinetic energy???

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

    Phrank (36.4kg) runs at 2.2 m/s and jumps onto a large stationary skateboard (2.3kg) on which Phrancine (22.7kg) is sitting. After the collision, Phrank, Phrancine, and the skateboard move at 1.3 m/s. Find the kinetic energy before and after the collision along with the percent difference.


    2. Relevant equations

    Kinetic energy before equals= 88.1 J
    Kinetic energy after= 51.88 J
    Percent difference= 88.1-51.88 / (88.1 + 51.88/2 ) x 100 = 51.75 WHAT ARE MY UNITS HERE???

    3. The attempt at a solution
    Need help with my units for percent difference! J is no correct!
     
    astru025, Oct 4, 2013
  2. jcsd
    Phys.org - latest science and technology news stories on Phys.org
  3. Oct 4, 2013 #2

    NobodyMinus

    User Avatar

    Percent difference is a unit-less number.
     
    NobodyMinus, Oct 4, 2013
  4. Oct 5, 2013 #3

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Insights Author
    Gold Member

    More precisely, it is a dimensionless number.
     
    haruspex, Oct 5, 2013
  5. Oct 5, 2013 #4

    Borek

    User Avatar

    Staff: Mentor

    And not percent? I can be horribly wrong, but I was always under impression that if it goes like

    [tex]\frac{3J}{2J}100\% = 150\%[/tex]

    ratio is dimensionless but the answer is in %.

    Not that % is a physical unit. Actually now that I think about it, I have no idea what % is.
     
    Borek, Oct 5, 2013
  6. Oct 5, 2013 #5

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Insights Author
    Gold Member

    You could certainly argue that dimensionless quantities nevertheless have units - whole numbers, percentages, millions.... But if I ask how many km/h a certain speed limit is the answer could be 50 - it would not, strictly speaking, be 50km/h. In the same way, if asked what percentage 3 is of 4 then the answer is 75, not 75% (which could legitimately be interpreted as 75/100 or 0.75). Does that make sense?
     
    haruspex, Oct 5, 2013
  7. Oct 5, 2013 #6

    Borek

    User Avatar

    Staff: Mentor

    More or less... not your fault, I am slow today :zzz:
     
    Borek, Oct 5, 2013
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Loading...
Similar Threads for units finding diff
When to convert units of temperature?
Why don't units have match for specific heat and mass
Unit vectors and block of ice
Finding the units in an algebraic equation for velocity