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Introducing vectors for engineering applications
Introducing vectors for engineering applications

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3.3 Vector subtraction in component form

When subtracting a vector visually it is first necessary to find the negative of the vector being subtracted by reversing its direction. Algebraically, a similar process is followed, but if we follow the standard rules of algebra, it is a much more intuitive process. For example, consider the vector expression a minus b where the vector bold b is subtracted from the vector bold a. We can make sense of this by writing the expression as

a plus left parenthesis negative b right parenthesis comma

where negative b is the negative of bold b. The negative of a vector has the same magnitude but the opposite direction, and for a vector a equals a sub one times i plus a sub two times j we say its negative is

equation sequence part 1 negative a equals part 2 negative left parenthesis a sub one times i plus a sub two times j right parenthesis equals part 3 negative a sub one times i minus a sub two times j full stop

With this in mind, we can say that to subtract vectors in component form, we subtract each component of one vector from the corresponding component of the other.

Subtracting vectors in component form

If a equals a sub one times i plus a sub two times j and b equals b sub one times i plus b sub two times j, then

a minus b equals left parenthesis a sub one minus b sub one right parenthesis times i plus left parenthesis a sub two minus b sub two right parenthesis times j full stop

In column notation, if a equals vector element 1 a sub one element 2 a sub two and b equals vector element 1 b sub one element 2 b sub two, then

a minus b equals vector element 1 a sub one minus b sub one element 2 a sub two minus b sub two full stop

Example 4 Calculating vector subtraction in component form

Let a equals five times i minus two times j and b equals negative i plus three times j. Find a minus b.

Solution

Subtracting the components of bold b from the corresponding components of bold a gives

equation sequence part 1 a minus b equals part 2 left parenthesis five times i minus two times j right parenthesis minus left parenthesis negative i plus three times j right parenthesis equals part 3 five times i plus i minus two times j minus three times j times equals six times i minus five times j full stop

Activity 16

Find the following vectors.

  • a.left parenthesis two times i plus j right parenthesis minus left parenthesis three times i plus two times j right parenthesis

  • b.left parenthesis three times i plus two times j right parenthesis minus left parenthesis negative two times i plus four times j right parenthesis

  • c.vector element 1 three element 2 four minus vector element 1 two element 2 negative one