Using our Rearranged Equations to Calculate the Answer Reinforcement Questions

1.

Rearrange this equation to isolate a, and then calculate its value:

begin mathsize 14px style x space equals space fraction numerator left parenthesis a d right parenthesis squared space plus space c over denominator r end fraction end style

Values of terms

begin mathsize 14px style d space equals space 1.2 space k g space space
c space equals space 0.8 space k g space space
r space equals space 2.4 space m cubed space space
x space equals space 290.73 space k g divided by m cubed end style

First, we can rearrange our equation starting by moving r across the equals sign by multiplying both sides by r:

begin mathsize 14px style x r space equals space left parenthesis a d right parenthesis squared space plus space c end style

Now we move c by subtracting it from both sides:

begin mathsize 14px style x r space minus space c space equals space left parenthesis a d right parenthesis squared end style

Then we remove the brackets by taking a square root of both sides:

begin mathsize 14px style square root of x r space minus space c end root space equals space a d end style

And finally divide both sides by d:

begin mathsize 14px style a space equals space fraction numerator space square root of x r space minus space c space end root over denominator d end fraction end style

Now we can input our values:

begin mathsize 14px style a space equals space fraction numerator square root of 290.73 space begin display style fraction numerator k g over denominator m cubed end fraction end style space cross times space begin display style fraction numerator 2.4 space m cubed over denominator 1 end fraction space end style minus space 0.8 space k g end root over denominator 1.2 space k g end fraction end style

Then we can cancel our units:

begin mathsize 14px style a space equals space fraction numerator square root of 290.73 space begin display style fraction numerator horizontal strike k g end strike over denominator horizontal strike m cubed end strike end fraction end style space cross times space begin display style fraction numerator 2.4 space horizontal strike m cubed end strike over denominator 1 end fraction space end style minus space 0.8 space k g end root over denominator 1.2 space horizontal strike k g end strike end fraction end style

Now we can calculate the numerator line doing the multiplication first:

begin mathsize 14px style a space equals space fraction numerator square root of 696.952 space k g end root over denominator 1.2 end fraction end style

And take the square root:

begin mathsize 14px style a equals space fraction numerator 26.4 space k g over denominator 1.2 end fraction end style

And finally divide by the denominator:

begin mathsize 14px style a space equals space 22 space k g end style

2.

Rearrange the following equation to isolate T2 and calculate its value.

begin mathsize 14px style q space equals space m C p space left parenthesis T subscript 1 space minus space T subscript 2 right parenthesis end style

begin mathsize 14px style C p space equals space 2 comma 500 space J divided by k g degree C space space
m space equals space space 1.39 space k g divided by s
T subscript 1 space equals space 50 space space degree C
q space equals space 135 comma 525 space J divided by s end style

Start by rearranging to isolate begin mathsize 14px style left parenthesis T subscript 1 space minus space T subscript 2 right parenthesis end style by dividing both sides by begin mathsize 14px style m C p end style :

begin mathsize 14px style fraction numerator q over denominator m C p end fraction space equals space T subscript 1 space minus space T subscript 2 end style

To move T1 across, we need to change the right-hand side to:

begin mathsize 14px style fraction numerator q over denominator m C p end fraction space equals space minus T subscript 2 space plus space T subscript 1 end style

And then subtract T1 from both sides:

begin mathsize 14px style negative T subscript 2 space space equals space fraction numerator q over denominator m C p end fraction space minus space T subscript 1 end style

Now we can input our figures:

begin mathsize 14px style negative T subscript 2 space space equals space fraction numerator 135 comma 525 space space begin display style J over s end style over denominator 1.39 space begin display style fraction numerator k g over denominator s end fraction end style space cross times space 2 comma 500 space begin display style fraction numerator J space over denominator k g space degree C end fraction end style end fraction space minus space 50 degree C end style

And do the calculations, cancelling our units:

begin mathsize 14px style negative T subscript 2 space space equals space fraction numerator 135 comma 525 space space begin display style J over s end style over denominator 1.39 space begin display style fraction numerator horizontal strike k g end strike over denominator s end fraction end style space cross times space 2 comma 500 space begin display style fraction numerator J space over denominator horizontal strike k g end strike space degree C end fraction end style end fraction space minus space 50 degree C end style

begin mathsize 14px style negative T subscript 2 space space equals space fraction numerator 135 comma 525 space space begin display style J over s end style over denominator space 3 comma 475 space begin display style fraction numerator J space over denominator s space degree C end fraction end style end fraction space minus space 50 degree C end style

begin mathsize 14px style negative T subscript 2 space space equals space 135 comma 525 space space fraction numerator horizontal strike J over denominator horizontal strike s end fraction space cross times space fraction numerator 1 over denominator space 3 comma 475 end fraction space fraction numerator horizontal strike s space degree C over denominator horizontal strike J end fraction space minus space 50 degree C end style

begin mathsize 14px style negative T subscript 2 space space equals space fraction numerator 135 comma 525 space space over denominator space 3 comma 475 space end fraction space minus space 50 degree C end style

And:

begin mathsize 14px style negative T subscript 2 space equals space 39 space minus space 50 degree C end style

And:

begin mathsize 14px style negative T subscript 2 space equals space minus 11 degree C end style

So:

 begin mathsize 14px style T subscript 2 space equals space 11 degree C end style

3.

Rearrange this equation to make begin mathsize 14px style rho subscript m end style the subject and calculate its value:

begin mathsize 14px style P subscript 1 space minus space P subscript 2 space equals space left parenthesis rho subscript m space minus space rho subscript R right parenthesis space g Z end style

begin mathsize 14px style P subscript 1 space equals space 450 comma 000 space N divided by m squared
P subscript 2 space equals space 102 comma 529.8 space N divided by m squared
g space equals space 9.81 space m divided by s space space
rho subscript R space equals space 250 space k g divided by m cubed space space
Z space equals space 20 space m end style

First, we can move gZ to the other side of the equation by dividing both sides by it:

begin mathsize 14px style fraction numerator P subscript 1 space minus space P subscript 2 over denominator g Z end fraction space equals space rho subscript m space minus space rho subscript R end style

And then make  begin mathsize 14px style rho subscript m end style the subject by adding begin mathsize 14px style rho subscript R end style to both sides:

begin mathsize 14px style space rho subscript m space equals space open parentheses fraction numerator P subscript 1 space minus space P subscript 2 over denominator g Z end fraction close parentheses space plus space rho subscript R end style

Now we input our numbers and units:

begin mathsize 14px style space rho subscript m space equals space open parentheses fraction numerator 450 comma 000 space begin display style N over m squared end style space minus space 102 comma 529.8 space begin display style N over m squared end style over denominator 9.81 space begin display style space m over s end style space cross times space 20 space m end fraction close parentheses space plus space 250 space fraction numerator k g over denominator m cubed end fraction space end style

Then we do the numerator and denominator calculations in brackets and cancel and combine the units:

begin mathsize 14px style space rho subscript m space equals space open parentheses fraction numerator 347 comma 470.2 space begin display style N over m squared end style space over denominator 196.2 space begin display style m squared over s end style end fraction close parentheses space plus space 250 space fraction numerator k g over denominator m cubed end fraction space end style

Now we complete the calculation in brackets and combine cancelling the units again:

begin mathsize 14px style space rho subscript m space equals space 1 comma 771 space fraction numerator N. s over denominator m to the power of 4 end fraction space plus space 250 space fraction numerator k g over denominator m cubed end fraction space end style

We then convert the derived unit N into the units it is derived from:

begin mathsize 14px style space rho subscript m space equals space 1 comma 771 space fraction numerator begin display style fraction numerator k g. m over denominator s squared end fraction end style left parenthesis s right parenthesis over denominator m to the power of 4 end fraction space plus space 250 space fraction numerator k g over denominator m cubed end fraction space end style

And cancel the units:

begin mathsize 14px style space rho subscript m space equals space 1 comma 771 space fraction numerator k g over denominator m cubed end fraction space plus space 250 space fraction numerator k g over denominator m cubed end fraction space end style

And finally, complete the calculation:

begin mathsize 14px style space rho subscript m space equals space 2 comma 021 space fraction numerator k g over denominator m cubed end fraction space end style

4.

Rearrange this equation to make x the subject and calculate its value.

begin mathsize 14px style 1 over U space equals space 1 over h subscript 1 space plus space x over k space plus space 1 over h subscript 2 end style

begin mathsize 14px style U space equals space 52.86 space W divided by m squared K
h subscript 1 space equals space 200 space W divided by m squared K space space space
k space equals space 250 space W divided by m K space
h subscript 2 equals space 169 space W divided by m squared K end style

First, we can start by moving the terms added to begin mathsize 14px style x over k end style across the equation by subtracting them from both sides.

begin mathsize 14px style 1 over U space minus space 1 over h subscript 1 space minus space 1 over h subscript 2 space equals space x over k end style

Then we can isolate x by multiplying both sides by k:

begin mathsize 14px style x space equals space k space open parentheses 1 over U space minus space 1 over h subscript 1 space minus space 1 over h subscript 2 close parentheses end style

Now we can input our numbers and units:

begin mathsize 14px style x space equals space 250 space fraction numerator W over denominator m K end fraction space open parentheses fraction numerator 1 over denominator 52.86 space begin display style fraction numerator W over denominator m squared K end fraction end style end fraction space minus space fraction numerator 1 over denominator 200 space begin display style fraction numerator W over denominator m squared K end fraction end style end fraction space minus space fraction numerator 1 over denominator 169 space begin display style fraction numerator W over denominator m squared K end fraction end style end fraction close parentheses
end style

And now we do the division calculations in the brackets. Notice how dividing 1 (taking a reciprocal) by the number make causes the units to invert:

begin mathsize 14px style x space equals space 250 space fraction numerator W over denominator m K end fraction space open parentheses 0.0189 space fraction numerator K m squared over denominator W end fraction space minus space 0.005 space fraction numerator K m squared over denominator W end fraction space minus space 0.0059 space fraction numerator K m squared over denominator W end fraction close parentheses
end style

And now the subtractions:

begin mathsize 14px style x space equals space 250 space fraction numerator horizontal strike W over denominator horizontal strike m K end strike end fraction space cross times space 0.008 space fraction numerator horizontal strike K m to the power of horizontal strike 2 end exponent over denominator horizontal strike W end fraction end style

Here the units cancel out to leave just m:

begin mathsize 14px style x space equals space 2 space m end style

Before we finish the lesson, we will cover another couple of maths topics that we might encounter in the Diploma lessons, the mole and interpolation. We start with the mole next.

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