# Frictional force- sign convention

1. Apr 22, 2012

### sphyics

Frictional force-- sign convention

For a horizontal spring mass system is the frictional force considered negative (-μmg)? i guess it will be negative; the minus sign used to convey the force is trying to impede the motion of block then with that guessing work; i think friction force will be always negative for a horizontal spring mass system. If i'm wrong please correct me :)

2. Apr 22, 2012

### Staff: Mentor

Re: Frictional force-- sign convention

IN a horizontal system, "+" usually means "to the right", and "-" means "to the left." Is your frictional force to the right or to the left?

3. Apr 22, 2012

### sphyics

Re: Frictional force-- sign convention

The frictional force is to the right; the case under consideration is: when the mass is stretched to the extreme right and let it go, as the mass moves towards the left under spring action, the friction force will try to impede the motion hence to the right.. But while deriving the work energy theorem for the particular case the friction force is taken as negative.. hence i'm confused!

4. Apr 22, 2012

### Staff: Mentor

Re: Frictional force-- sign convention

Work is positive when the force is in the direction of motion (both + or both - in the case of one-dimensional motion) and negative when the force is opposite the direction of motion (one + and the other -).

5. Apr 22, 2012

### sphyics

Re: Frictional force-- sign convention

GOT IT!! i made a mistake while calculation; i mistook force as +ve and negative whereas i was supposed to take the work due to frictional force negative :)