Uses of the Scalar Product (CIE A Level Maths: Pure 3)

Revision Note

Test yourself
Amber

Author

Amber

Last updated

Did this video help you?

Uses of the Scalar Product

This revision note covers several applications of the scalar product for vectors – namely, how you can use the scalar product to:

  • find the angle between vectors or lines
  • test whether vectors or lines are perpendicular
  • find the closest distance from a point to a line

How do I find the angle between two vectors?

 

  • Recall that a formula for the scalar (or ‘dot’) between vectors bold a and bold b is

bold a bold times bold b equals open vertical bar bold a close vertical bar open vertical bar bold b close vertical bar cos invisible function application theta

    • where theta is the angle between the vectors when they are placed ‘base to base’
      • that is, when the vectors are positioned so that they start at the same point
    • We arrange this formula to make cos space theta the subject:
    • To find the angle between two vectors
      • Calculate the scalar product between them
      • Calculate the magnitude of each vector
      • Use the formula to find cos space theta
      • Use inverse trig to find theta

How do I find the angle between two lines?

  • To find the angle between two lines, find the angle between their direction vectors
    •  For example, if the lines have equations bold r equals bold a subscript 1 plus s bold d subscript 1 and bold r equals bold a subscript 2 plus t bold d subscript 2, then the angle theta between the lines is given by

theta equals cos to the power of negative 1 end exponent open parentheses fraction numerator bold d subscript 1 bold times bold d subscript 2 over denominator open vertical bar bold d subscript 1 close vertical bar open vertical bar bold d subscript 2 close vertical bar end fraction close parentheses

 

How do I tell if vectors or lines are perpendicular?

  • Two (non-zero) vectors bold a and bold b are perpendicular if, and only if, bold a bold times bold b equals 0
    • If the a and b are perpendicular then:
      • theta equals 90 degree rightwards double arrow cos space theta equals 0 rightwards double arrow open vertical bar bold a close vertical bar open vertical bar bold b close vertical bar cos space theta blank equals 0 rightwards double arrow bold a bold times bold b equals 0
    • If  bold a bold times bold b equals 0 then:
      • open vertical bar bold a close vertical bar open vertical bar bold b close vertical bar cos space theta blank equals 0 rightwards double arrow cos space theta equals 0 rightwards double arrow theta equals 90 degree rightwards double arrow a and b are perpendicular
    • For example, the vectors 2 i minus 3 j plus 5 k and negative 4 i minus j plus k  are perpendicular since

open parentheses 2 i minus 3 j plus 5 k blank close parentheses times open parentheses negative 4 i minus j plus k close parentheses equals 2 cross times open parentheses negative 4 close parentheses plus open parentheses negative 3 close parentheses cross times open parentheses negative 1 close parentheses plus 5 cross times 1 equals negative 8 plus 3 plus 5 equals 0

How do I find the shortest distance from a point to a line?

  • Suppose that we have a line l with equation bold r equals bold a plus t bold d  and a point P not on l
  • Let F be the point on l which is closest to bold italic P (sometimes called the foot of the perpendicular)
    • Then the line between F and P will be perpendicular to the line l
  • To find the closest point F
    • Call bold f equals OF with rightwards arrow on top and bold p equals stack O P with rightwards arrow on top
    • Since F lies on l, we have bold f equals bold a plus t subscript 0 bold d, for a unique real number t subscript 0
    • Find the vector stack bold italic F bold italic P with rightwards arrow on top using bold p minus bold f
    • stack bold italic F bold italic P with rightwards arrow on top is perpendicular to bold italic d so form an equation using open parentheses bold p minus bold f close parentheses times bold d equals 0
    • Solve this equation to find the value of t subscript 0
    • Use the value of t subscript 0 to find bold f
  • The shortest distance between the point  and the line  is the length
  • Note that the shortest distance between the point and the line is sometimes referred to as the length of the perpendicular

7-3-4-foot-of-the-perpendicular

Worked example

7-3-4-uses-of-scalar-product-we-solution-part-1

7-3-4-uses-of-scalar-product-we-solution-part-2

Examiner Tip

It can be easier and clearer to work with column vectors when dealing with scalar products.

You've read 0 of your 5 free revision notes this week

Sign up now. It’s free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Amber

Author: Amber

Expertise: Maths

Amber gained a first class degree in Mathematics & Meteorology from the University of Reading before training to become a teacher. She is passionate about teaching, having spent 8 years teaching GCSE and A Level Mathematics both in the UK and internationally. Amber loves creating bright and informative resources to help students reach their potential.