# When is friction and weight parallel positive and negative?

1. Jan 4, 2010

### stargirl17

1. The problem statement, all variables and given/known data

When writing Fnet=ma you are supposed to list the forces in Fnet
-Ffriction + Weightparallel= ma

how do you know when to put the negative or positve sign in front of the values? like on inclines and horizontal surfaces

2. Relevant equations

3. The attempt at a solution

2. Jan 4, 2010

### JaWiB

You get to choose which direction is positive and which is negative. Typically, for inclines you choose axes that are aligned with the incline, and the direction down the ramp is most often negative, but you can choose down the ramp to be positive and up to be negative just as well.

3. Jan 4, 2010

### stargirl17

ok so if a box is going down an incline, friction can be positive, but then weight parallel and weight perpendicular have to be negative right?

4. Jan 4, 2010

### ideasrule

Weight parallel has to be negative, yes, but why does the perpendicular weight have to be negative? You can define it as positive if you want.

5. Jan 4, 2010

### stargirl17

yeah your right. It depends if you make the normal force positive. They have to balance each other out. I understand now. thank you so much for your help, it was great :)