Wondering about the signs or coordinate system for forces?

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Homework Statement

What are the signs of the forces (positive + or negative -) acting on both situations?

Situation 1:

Situation 2:

Homework Equations

Not really necessary

The Attempt at a Solution

I'm not sure about Situation 1 at all, but I think I got Situation 2:

If I have this coordinate system:

On m1:
Fg1 = -
T = +

On m2:
Fg2 = -
T = +

On m3:

Fg3 = -
T = +

Is that right? Now how would I go about doing Situation 1? Would I have to employ a rotated coordinate system like:

Can anyone list the forces like that?

EDIT: By the way, this is so that I can implement them properly in my equations. For example, Fnet for m1 would then be T - Fg1 instead of T + Fg1. Or is there a better way of doing that instead of using coordinate systems?

 

[Andrew Mason]

You can use any co-ordinate system you want so long as you use it consistently. Directions for + and - is just a convention.

I would stick with the up=+ right=+. To do the inclined plane problem, you have to find the components of force horizontal to and perpendicular to the plane. But that does not require changing the co-ordinate system.

AM

 

[page123]

You can use any co-ordinate system you want so long as you use it consistently. Directions for + and - is just a convention.

I would stick with the up=+ right=+. To do the inclined plane problem, you have to find the components of force horizontal to and perpendicular to the plane. But that does not require changing the co-ordinate system.

AM

Interesting, so what would the signs be for all the forces on that inclined problem, if there were two tensions and the normal force of gravities?

 

[Andrew Mason]

Interesting, so what would the signs be for all the forces on that inclined problem, if there were two tensions and the normal force of gravities?

You have to work that out.

AM

 

[page123]

You have to work that out.

AM

Are you sure about keeping the coordinate system consistent? Because on one pulley system where the mass is greater than the other one I saw T - Fg1 and Fg2 - T for each Fnet. If you had a consistent coordinate system the Ts would both be positive and the Fgs would both be negative. So: T - Fg1 and T - Fg2, but that wouldn't get the right answer?