Changing the Subject of a Formula
v = u + at is a formula with v as the subject. Sometimes we want to change the subject. Another way of saying this is ‘write t in terms of v, a, and u’.
In this case we are changing the subject to t.
We use the same rules as equations:
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v = u + at |
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(– u) |
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v – u = at |
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(÷ a)
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v – u = t
a |
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Normally we put the subject on the left, so: |
t = v – u
a |
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Many formulae can be rearranged in this way using the rules of equations.
However there are two special situations, as described below.
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Special Situations
1. When the new subject is inside a square root sign.
| For example: |
p = t .gif)
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(÷ t)
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p/ t =
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(square both sides)
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p2 = q |
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t2
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q |
= p2 |
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t2 |
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Note: We can remove the square root sign by making it the subject, then squaring both sides of the formula.
2. When the new subject is squared.
| For example: |
y = p2 – 3
2 |
(x 2)
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2y = p2 – 3 |
(+ 3) |
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2y + 3 = p2 |
(square root) |
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p =  |
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Note: We make the squared term the new subject, then square root.
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