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EXAMPLE 4<br />

Calculating a specific direction angle<br />

For the vector OP ! 12V2, 4, 52, determine the direction cosine and the<br />

corresponding angle that this vector makes with the positive z-axis.<br />

Solution<br />

We can use the formula to calculate g.<br />

cos g <br />

5<br />

V12V22 2 142 2 152 2<br />

5<br />

V49 5<br />

7 0.7143<br />

and g 135.6 °<br />

Examining Vector Projections<br />

Thus far, we have calculated scalar projections of a vector onto a vector. This<br />

computation can be modified slightly to find the corresponding vector projection<br />

of a vector on a vector.<br />

The calculation of the vector projection of on is just the corresponding scalar<br />

b ! a ! b !<br />

b !<br />

projection of a ! on b !<br />

multiplied by . The expression is a unit vector pointing<br />

in the direction of b ! @ b ! @<br />

@ b ! @<br />

.<br />

A<br />

A<br />

a<br />

a<br />

u<br />

O<br />

b<br />

N B<br />

Scalar projection of a on b<br />

u<br />

O<br />

b N B<br />

Vector projection of a on b<br />

Vector Projection of a ! on b !<br />

vector projection of a ! on<br />

(scalar projection of a ! b!<br />

on b !<br />

) (unit vector in the direction of b !<br />

)<br />

a a! !<br />

# b<br />

@ b ! ba b!<br />

@ @ b ! b<br />

@<br />

a! !<br />

# b<br />

@ b ! @ b! 2<br />

a a! !<br />

# b<br />

, b ! 0 !<br />

b ! ! b b !<br />

# b<br />

396 7.5 SCALAR AND VECTOR PROJECTIONS<br />

NEL

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