Subject of formula is a very important concept in physics since it will enable you to manipulate equations properly.
If you have the equation
s = ut + 0.5 at2
and you want to calculate the acceleration a, how will you do it?
You will have to write an equation of a in term of the other quantities like
a =
You will do this by moving the different quantities around until only a is left on one side. This is called making a the subject of formula.
We will start by studying four simple rules and then I will teach you how to use them to manipulate simple equations.
Rule 1
If you have the equation A + B = C and you want to have A in term of B and C what will you do?
A + B = C
You want A to remain on the left-hand side, as a result you will want to move + B to the right-hand side.
The + B will have to change to – B when transferred to the right-hand side.
Hence A + B = C
becomes
A = C - B
The first rule is that a + on one side becomes a + on the other side.
Rule 2
If you have A – B = C and you want to have A in term of B and C what will you do ?
A – B = C
You want A to remain on the left-hand side and move – B to the right-hand side.
The – B will have to change to + B when transferred to the right-hand side.
Hence A – B = C
becomes
A = C +B
The second rule is that a – on one side becomes a + on the other side.
Rule 3
If you have A*B = C and you want to write A in term of B and C what would you do?
A*B = C
If you want A to remain on the left-hand side then you would have to move *B to the other side.
The *B will have to change to /B when transferred to the right-hand side.
Hence A*B = C
becomes
A = C/B
The third rule is that a * on one side becomes a / on the other side.
Rule 4
If you have A/B = C and you want to write A in term of B and C what would you do?
A/B = C
If you want A to remain on the left-hand side you will have to move /B to the other side.
The /B will have to change to *B when transferred to the other side.
Hence A/B =C
becomes
A = C*B
The fourth rule is that a / on one side becomes a * on the other side.
