# I need help calculating the frictional force

#### moso13

to calculate the work done by friction. I have that the kinetic coefficient is .09 and I know delta x and i know for the problem the other part of the eq is cos(180).I also know mass is 215 kg. and acceleration is .131
So it is Wf= Ff*1.77*cos180
In the worked out solutions my professor but the Ff as 177 but I can't figure out how he got that. Thank you.

#### ChipB

PHF Helper
I get $F_f = \mu m g = 189 N$. Could it be that either (a) the mass is actually 200Kg, not 215 Kg, or (b) the mass is sliding on an incline, in which case the normal force would be $mg \cos \theta$ ?

#### moso13

I get $F_f = \mu m g = 189 N$. Could it be that either (a) the mass is actually 200Kg, not 215 Kg, or (b) the mass is sliding on an incline, in which case the normal force would be $mg \cos \theta$ ?
Oh the mass is on an incline! I forgot to mention that. But how would that change my calculation? Its not asking about the normal force?

#### ChipB

PHF Helper
Friction force = coefficient of friction times the normal force. The normal force is the component of the force between the body and the sliding surface (i.e. its weight) that is perpendicular (normal) to the surface. Put it all together and you get:

$F_f = \mu mg \cos \theta$

Hope this helps.