# How to find magnitude of two forces

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1. Feb 15, 2015

### janex

Two forces are given and are applied to a car in an effort to accelerate it: 414N at 9 degrees and 340N at 25 degrees. What is magnitude of the resultant of the two forces? Answer in units of N.

So I've added together X= (414cos9) + (340cos25), and got -40.
For the Y, Y= (414sin9) + (340sin25)=125.
I took square root of (-40)^2 + (125)^2, and got 131.24.

However, that is not the correct answer. What am I doing wrong? Help please?!

2. Feb 15, 2015

### TSny

Note the angles are in degrees, not radians.

3. Feb 15, 2015

### SteamKing

Staff Emeritus
You've added two positive quantities together and obtained a negative result. How?

4. Feb 15, 2015

### phinds

Well you are computing the right values to add together for X and also for Y but the answers you get for those computation are just weird.

5. Feb 15, 2015

### janex

Steamking, 414 x cos 9 degrees = - 377.2.

6. Feb 15, 2015

### TSny

Check your calculator to make sure it is in degree mode.

7. Feb 15, 2015

### janex

So what you are saying Tsny is that I should convert the answers to radians?

8. Feb 15, 2015

### TSny

Keep the angles in degrees and just switch the angle mode of your calculator to degrees instead of radians. Your calculator is assuming that your angles are in radians.

9. Feb 15, 2015

### janex

I've switched the mode to degrees, but still am getting the wrong answer. :(

10. Feb 15, 2015

### TSny

what does your calculator now give you for cosine of 9 degrees?

11. Feb 15, 2015

### janex

.9876883

12. Feb 15, 2015

### TSny

OK good. Was a diagram included in the problem? Are the two angles measured from the x axis? Are both angle above the x axis, or is one angle above the axis and the other angle below the axis?

13. Feb 15, 2015

### janex

340 N @ 25 degrees is below x-axis. 414 N @ 9 degrees I above x-axis.

14. Feb 15, 2015

### TSny

Are the Y components of both forces positive? If not, which force has a negative Y component?

15. Feb 15, 2015

### janex

Yes

16. Feb 15, 2015

### TSny

The 340 N points below the x axis. Draw this force and draw its Y component. Does the Y component point up or down?

17. Feb 15, 2015

### janex

I
It is pointing up

18. Feb 15, 2015

### TSny

19. Feb 15, 2015

### janex

Ye
I think once I get a good look at this tomorrow, it'll make better sense. As far as I can see, the vector does move in a negative direction towards F2.

20. Feb 15, 2015

### janex

Thanks for all your help TSny!