Let |u| = 10 at an angle of 45° and |v| = 13 at an angle of 150°, and w = u + v. What is the magnitude and direction angle of w?
|w| = 9.4; θ = 72.9°
|w| = 9.4; θ = 107.1°
|w| = 14.2; θ = 72.9°
|w| = 14.2; θ = 107.1°
Answers
Recall that for two vector x and y making an angle θ with each,
x • y = ||x|| ||y|| cos(θ)
If we replace y with x, we see that
x • x = ||x|| ||x|| cos(0) = ||x||² ⇒ ||x|| = √(x • x)
Using the last identity, the magnitude of w is
||w|| = √(w • w)
but since w = u + v, we have
w • w = (u + v) • (u + v)
The dot product distributes over sums and is commutative, so
w • w = (u • u) + (u • v) + (v • u) + (v • v)
… = ||u||² + 2 (u • v) + ||v||²
… = ||u||² + 2 ||u|| ||v|| cos(θ) + ||v||²
If u has a direction of 45° with the positive x-axis, v has a direction of 150°, then the angle between u and v is |45° - 150°| = 105°. So,
||w|| = √(||u||² + 2 ||u|| ||v|| cos(150°) + ||v||²)
… = √(10² + 2 • 10 • 13 cos(150°) + 13²)
… ≈ 14.2
Using the parallelogram rule for vector addition (see attached sketch), the sum of the angle between w and u and 45° is equal to the direction of w.
If φ is the angle between w and u, then
w • u = ||w|| ||u|| cos(φ)
… = 14.2 • 10 • cos(φ)
but we also have
w • u = (u + v) • u
… = (u • u) + (v • u)
… = ||u||² + ||u|| ||v|| cos(105°)
… = 10² + 10 • 13 • cos(105°)
… ≈ 66.4
Then
14.2 • 10 • cos(φ) ≈ 66.4
cos(φ) ≈ 0.467
φ ≈ 62.1°
and so the direction of w is 62.1° + 45° ≈ 107.1°.
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