help with physics homework please?

Question

How do i find the magnitude of friction force when i know: μs μk and Mass?

its supposed to be if it is a block on a table at rest. and the only other force acting on it is gravity!

and then how do i change it if it is on an incline?? ???


I am so confused? is there a certain equation to use?

Answer 1

Static frictional force = μs * Fn (where Fn = the weight component directed perpendicular to the friction).

Kinetic frictional force = μk * Fn
Fn will equal mg cos (angle of ramp). If there's no angle (like on a table), then Fn is equal to weight (mg).

So, a 2 kg block on a 30 degree ramp with a μs of .5 has a frictional force of:

Ff = μs * Fn
Fn = mg cos 30 = 2 * 9.8 * 0.866 = 16.974N
Ff = μs * Fn
Ff = .5 * 16.974N = 8.487N