# Force vectors acted on by two forces

kay
Sorry to disturb, but could we have found the magnitude of the resultant by using the parallelogram law?

ehild
Homework Helper
Sorry to disturb, but could we have found the magnitude of the resultant by using the parallelogram law?
Yes, you can.

The magnitude of the sum of wo vectors $\vec c= \vec a+\vec b$ is
$|c|=\sqrt{ ( \vec a + \vec b)^2}=\sqrt {|a|^2 + |b|^2 + 2|a||b| \cos (θ) }$
which is the length of the "long" diagonal of the parallelogram made by vectors $\vec a$ and $\vec b$. θ is the angle between the vectors. You can also apply the cosine law for the blue triangle.

ehild

#### Attachments

• 10.5 KB Views: 261
Last edited: