# Three forces act on a moving object

## Homework Statement

Three forces act on a moving object. One force has a magnitude of 80.0 N and is directed due north. Another force has a magnitude of 60.0 N and is directed due west. What must be the magnitude and direction of the third force, such that the object continues to move with a constant velocity?

Force 1 = 80.0 N due north.
Force 2 = 60.0 N due west.

## Homework Equations

a^2 + b^2 = c^2
Theta = tan^-1(fy/fx)

## The Attempt at a Solution

First I started by plotting the problem on a cartesian plane. I then took my known quantities of 80.0 N due north (along the positive y axis) and 60.0 N due west (along the negative x axis) and used pythag to find the net force, the answer of which was 100 N.

I then attempted to find the direction using Theta = tan^-1(fy/fx) = tan^-1(80/60) = 53.13.

My question is, am I right in assuming that I must counteract the first two forces with the third to ensure a constant velocity? If so I would appreciate any help in this regard.

## Answers and Replies

lewando
Homework Helper
Gold Member
Careful with your direction. 53.13 degrees with respect to what? West of North? North of West? Otherwise, your assumption is correct.

Careful with your direction. 53.13 degrees with respect to what? West of North? North of West? Otherwise, your assumption is correct.

Thanks. I will be sure to always define that in future So the next step in my problem having confirmed my original assumption (thanks again) would be to determine the magnitude of the third force and it's direction:

The net force I worked out to be 100 N and the direction to be 53.13 degrees North of West So the magnitude would have to be 100 N in the opposite direction being 53.13 degrees South of East?

I appreciate all help as I have just begun studying again and I want to fully grasp each concept as I go along.

Thanks.

lewando
Homework Helper
Gold Member
So the magnitude would have to be 100 N in the opposite direction being 53.13 degrees South of East?

Yep. 