# Homework Help: How to use friction equations

1. Dec 13, 2012

### HelloImAsh

1. The problem statement, all variables and given/known data
The coefficient of sliding friction between two materials is 0.35. A 5.0 kg object made of one material is being pulled along a table made of another material. What is the force of friction?

I'm fairly certain I solved the problem correctly, the thing is, I should be getting a negative force as the answer (because I defined forward as positive), and I'm getting a positive force. Could someone please point out which part of my solution is incorrect.

2. Relevant equations
F1 is force of sliding friction, F2 is normal force, and μ is the coefficient of sliding friction:
F1=μF2

3. The attempt at a solution
F1=μF2
=0.35(mg)
=17.15N
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Dec 13, 2012

### SammyS

Staff Emeritus
The equation F1=μF2 only gives the magnitude of the frictional force. The force of friction, F1, is in a direction perpendicular to the normal force, F2 , so that equation cannot be true in a vector sense.

The force of friction is in such a direction as to oppose the relative motion of the two objects.