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!

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

  1. Oct 4, 2013 #1
    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!
     
  2. jcsd
  3. Oct 4, 2013 #2
    Percent difference is a unit-less number.
     
  4. Oct 5, 2013 #3

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

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

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    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?
     
  7. Oct 5, 2013 #6

    Borek

    User Avatar

    Staff: Mentor

    More or less... not your fault, I am slow today :zzz:
     
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: What are my units when finding % diff. In kinetic energy?
  1. Finding Kinetic energy (Replies: 5)

Loading...