# Thermal/mechanical properties of matter

1)Explain why a correction must be applied for displacement of air in accurate weighing with a common balance.

Calculate the percentage error which would arise through neglect of this correction in weighing water with platinum weights of density 2.15x10^4 in air density 1.22. (units are kg and m)

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I'm not really sure what to do with this question, is the first part involving the non-uniform distribution of air and the resulting varying pressure or something? hmm. I've scanned a textbook i have access to, it's not very well up on thermal stuff and I can't get to library for a few days. beh. As for the second part, hints are welcome :uhh:

dextercioby
Homework Helper
Are u sure it's "thermal stuff" and not Archimede's buoyant force...?

Judging from the second problem,it would seem that way...

Daniel.

yeah me too, thats just what the problem sheet was titled. makes it a little more confusing. I still don't get it though.. :|

dextercioby
Homework Helper
You mean "thermal/mechanical properties of matter"...?Neglecting complete vagueness of the phrase,i think the "mechanical" part would account for Archimede's buoyant force...

How about asking the dude/chick who gave the problem what on Earth was he/she meaning...

Daniel.

I am itching to give it a shot....

subscripts a, w and p correspond to air, water and platinum resp.
Buoyant force act on both sides of the scale.

$$v_w \rho _w - v_w\rho_a = v_p\rho_p - v_p\rho_a$$

$$\frac{v_w}{v_p} = \frac{(\rho_p - \rho_a)}{ \rho_w - \rho_a$$

real weight = $$\rho_w v_w$$

Apparent weight = $$v_w\rho_w - (v_w\rho_a - v_p\rho_a)$$

% diff = $$\frac{\rho_a}{ \rho_w}\left( \frac { \rho_p - \rho_w}{\rho_p - \rho_a} \right)$$

Does it sounds right?

edit: I was attempting to write the ratio of $$v_w/v_p$$ by rearranging the first equation. It shows up OK in my preview page.
But shows error after submitting.

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