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Mechanics Relative Speed and Velocity continued Relative Velocity The calculations in the examples on the previous page were very simple because we considered bodies moving along the same straight line. To find the velocity of one body relative to another when they move in different directions (that is, not along the same straight line) we use the same principle but the subtraction will have to be a vector subtraction. For example, consider again the two cars but moving as described below.
To find the velocity of car A relative to
car B we must perform the vector subtraction
To subtract vector B from vector A we simply add the opposite of vector B to vector A, as shown below.
From this we find that
If you imagine yourself to be in car B, watching car A, you will agree that this result is reasonable.
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= 20ms-1 east)
= 15ms-1 north)
.
is a velocity of magnitude 25ms-1 in a direction at
36·9° south of east.