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Homogenity physics homework

  • Thread starter MemoNick
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  • #1
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1. So for today's exam, I was given this equation, and I was required to get the value and units of K:

T4 = (4K∏4)/g2 - (8d∏4)/g2

I had to follow an experiment, plot the results, get the gradient. After getting the value of g, I was required to find the value of K, and its units.

T4 has units s4, d has units m, g is gravitational acceleration, hence I believe it is m.s-2



2. The problem lies in the fact that I don't believe I was given a homogeneous equation, hence I couldn't provide the units. Any help? Note that the units are enough for me, I managed to solve for K (correctly, I believe).

Thanks in advance :)
 

Answers and Replies

  • #2
662
307
You can only add or subtract things with same units.
 
  • #3
12
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So you confirm the equation is not homogeneous?
 
  • #4
662
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I neither confirm nor deny, I merely point out the obvious. What then is ∏? Or did you mean [itex]\pi[/itex]?
 
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  • #5
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Well, after working it out, the left side is this: T4 has units s4.

For the K part, I can't work it out without knowing whether it's homogeneous or not. So I go where there's d.

(8d∏4)/g2 would then become:
Numerator: m
Denominator: m2.s-4

Therefore this part would be s4.m-1

For homogeneity with addition/subtraction, all parts have to be the same. These two parts aren't the same, evidently. Any confirmation would be very much appreciated, so I could e-mail my invigilators.
 
  • #6
662
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What is the experiment?
 
  • #7
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That should be a pi :) I couldn't find the symbol :P
 
  • #8
662
307
Nope, it doesn't look homogenous. What's the Experiment?
You sure d is in meters and not m^2 ?
 
  • #9
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The experiment isn't very important per-se, but for the record, there are two stand clamps, with a wire attached to both, forming a triangle. At the bottom, there's a pendulum, and d is the distance between the two stands. So yeah, definitely in meters. It's also given, but because of copyright issues, I'm afraid I'm not allowed to take a picture of it and upload it.
 
  • #10
gneill
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Could be a typo in the given equation; I suspect that the 'd' in the second term should have been ##d^2##.

Even if the second term has a typo, the terms should individually have the same units as the LHS of the equation. So you can still determine what k should be and even repair the typo!
 
  • #11
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While I did as you said, gneill, I couldn't ascertain which was the right one - the 'd' part or the LHS.
 
  • #12
gneill
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While I did as you said, gneill, I couldn't ascertain which was the right one - the 'd' part or the LHS.
I think you can be pretty certain the LHS would consist of a single variable, and that as missed typos go, dropping a square on a variable in the midst of a term is more likely than dropping a variable on the LHS.
 
  • #13
662
307
Well, I haven't got the derivation of the formula yet but with off-hand information the formula's probably wrong. When you put d=0 the bifilar pendulum becomes a simple pendulum the formula should be reduced to a simple one.
Making $$k= 4 l^2$$ from the standard formula of time period. Formula's probably missing a square factor on d, will confirm it and let you know.
Whoops gneil got there before me....
 
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  • #14
12
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Thanks a lot for your help :) I really appreciate it!
 

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