# Homework Help: Vector Addition Problem - Statics

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1. Jan 19, 2015

### Jack_M

1. The problem statement, all variables and given/known data
Find the magnitude and direction of the resultant force Fr=F1+F2+F3 by first finding F'=F1+F2 then Fr=F'+F3

known values are in the link
2. Relevant equations
Basic vector addition. Law of cosines. Law of sines.

3. The attempt at a solution
Here's my attempt. I'm not convinced by my answer.
http://imgur.com/LcrFApu

Last edited by a moderator: Jan 19, 2015
2. Jan 19, 2015

### Quantum Defect

Another way of calculating the force is using x and y components. Add up all of the x components and all of the y-components of the three vectors to get the x and y components of the final vector. This saves you the trouble of calculating an intermediate vector.

x component = magnitude * cos (theta) -- theta measured from +x axis; y-component = magnitude * sin (theta) -- again theta measured from +x axis.

Last edited by a moderator: Jan 19, 2015
3. Jan 19, 2015

### BvU

Hello Jack, welcome to PF :)

F' is just fine.
Fr magnitude is fine too. Check the angle calculation. From your drawing you can already see that 29.12 degrees looks to be too low.

Of course, Quantum D gives a good alternative -- that should be equivalent, but I don't know if that's what the exercise wants you to do.

4. Jan 19, 2015

### Quantum Defect

I get essentially the same numbers that are in the box using x, y vectors: 19 degrees, 29.6 N

5. Jan 19, 2015

### Jack_M

QD, I would've much rather have done it using component method, however my professor would take off points.

BvU, are you referring to the 29.67N as being the correct magnitude for Fr and 19.12 degrees being incorrect? Could it be 19.12 degrees from the vertical?

6. Jan 19, 2015

### Jack_M

That's reassuring. Thanks for checking!

7. Jan 19, 2015

### Quantum Defect

My final x components were: +9.7 N x^ direction, +28 N y^ direction -- using inverse tangent gives me 19 degrees from y-axis, as you conclude above.

Last edited: Jan 19, 2015
8. Jan 19, 2015

### BvU

To reassure you both: I've been brainwashed to zero degrees $\equiv$ positive x-axis direction. So I find 1.237 (radians, that is. The one and only reasonable unit for angles :) -- did I also say the brainwashing included $2\pi$ for a full circle ?) And I really am a physicist !

So we all agree and Jack can shed his uncertainty on this subject.