# Math help needed

dandogg
hi i was wondering if there was a formula to find the angle between two forces
i.e. two forces of 14lb and 30lb act on a body forming an obtuse angle with each other having a resulting force of 20lbs

## Answers and Replies

Gale
Its a trig problem. Draw a picture of the two forces and the resultant force. create two triangles, and label all the sides. then use law of cosines to find one angle, and then the other angle. sum the two angles for the total angle between the two forces.

whozum
Theres two ways you can add the two forces to get a net force of 20lbs. One is acute and one is that same angle + 90 deg (obtuse).

You'll want to use vector addition either way.

dandogg
so if i make a triangle, the largest force of 30would be the hypotenuse and the magnitude and the
other force would be the other two sides?

Last edited:
Science Advisor
Homework Helper
No, strictly speaking, the term "hypotenuse" only applies to right triangles. It is true, of course, that in your triangle, the sides will have "lengths" 14, 20, and 30. Since the 14 and 30 pound forces are the ones adding, you want the angle between those sides.
Use the cosine law to find that angle.