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

  • Thread starter astru025
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  • #1
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What are my units when finding % diff. In kinetic energy???

Homework Statement



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.


Homework 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???

The Attempt at a Solution


Need help with my units for percent difference! J is no correct!
 

Answers and Replies

  • #2
Percent difference is a unit-less number.
 
  • #3
haruspex
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More precisely, it is a dimensionless number.
 
  • #4
Borek
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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.
 
  • #5
haruspex
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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.
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?
 
  • #6
Borek
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Does that make sense?
More or less... not your fault, I am slow today :zzz:
 

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