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the overall magnitude would be equal to the sum of their individual magnitudes. If<br />

they pulled in opposite directions, the overall magnitude would be their difference.<br />

Two force vectors acting<br />

in the same direction<br />

Two force vectors acting<br />

in opposite directions<br />

f 1 f 2<br />

f 1<br />

f 2<br />

EXAMPLE 1<br />

Selecting a strategy to determine the magnitude of a resultant vector<br />

Given vectors and such that the angle between the two vectors is<br />

and @ b ! a ! b !<br />

@ 2, determine a ! b ! 60°, 0 a ! 0 3,<br />

.<br />

Solution<br />

If it is stated that the angle between the vectors is u, this means that the vectors are<br />

placed tail to tail and the angle between the vectors is u. In this problem, the angle<br />

between the vectors is given to be 60°, so the vectors are placed tail to tail as shown.<br />

A<br />

b<br />

D<br />

608<br />

a<br />

B<br />

b<br />

D<br />

a + b<br />

608<br />

1208<br />

A<br />

B<br />

a<br />

) AB ) = ) a ) = 3 and ) AD ) = ) b ) = 2<br />

b<br />

C<br />

To calculate the value of draw the diagonal of the related parallelogram.<br />

From the diagram,<br />

Note that the angle between<br />

and BC ! AB ! @ a ! <br />

BC ! b ! @ ,<br />

a ! b ! AC ! .<br />

AB !<br />

is 120°, the supplement of 60°.<br />

Now, @ a ! b !<br />

@ a ! b ! @ 2 0 a ! 0 2 @ b ! @ 2 2 0 a ! 0 @ b ! @ cos1ABC2<br />

@ 2 3 2 2 2 2132122cos 120°<br />

@ a ! b ! @ 2 13 2132122 a 1<br />

@ a ! b ! 2 b<br />

@ 2<br />

Therefore, @ a ! b ! 19<br />

@ V19 4.36.<br />

(Cosine law)<br />

(Substitution)<br />

When finding the sum of two or more vectors, it is not necessary to draw<br />

a parallelogram each time. In the following, we show how to add vectors<br />

using the triangle law of addition.<br />

284<br />

6.2 VECTOR ADDITION<br />

NEL

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