how do we find the magnitude of the force of friction if we have static friction and kenetic friction?

Answer 1

Static friction force is that maximum force acting on a body just before it starts to slide. f(S) = sN; where f(S) is the static friction, s is the coefficient of static friction between a body and the something its resting on, and N is the normal weight of the body resting on something. The normal weight is the weight acting perpendicular to the something the body is resting on (like a ramp).

N = Wcos(angle); where W is the weight of the body when flat on the ground and angle is the angle between the horizontal and the tilt of whatever the body is sitting on (like a ramp). In the extreme, when the ramp is flat on the ground cos(0) = 1.00; so that N = Wcos(0) = W.

You can find s by raising one end of the ramp with the body resting on it until the body just starts to slide. At that point measure the angle of the ramp with respect to the horizontal. s = sin(angle). For example, if the body started to slide when the ramp was at 45 deg with the floor, s = sin(45) = .707; higher angles mean greater values of s and the body is stickier to the ramp.

The kinetic friction force has the same equation, but we use r the rolling or kinetic coefficient of friction rather than s. We can use the body and ramp to find this as well, but we need to do something other than simply tilt the ramp.

In this experiment, we start with a tilted ramp at an angle greater than the angle we used to get s and time how long the body takes to slide down the length (L) of that ramp. The body starts down the ramp with zero inital velocity (v0) and takes t seconds to travel L distance; so the average velocity (v = L/t) and average accleration (a) is v/t =(L/t)/t = L/t^2.

F(r) = ma = m(L/t^2) = rWsin(angle); so r = m(L/t^2)/(Wsin(angle) = m(L/t^2)/(mg sin(angle)) = (L/t^2)/(g sin(angle)) and we can solve for the kinetic coefficient of friction. Notice that r = a/(g sin(angle)) so the units cancel out and r, like s, is unitless as it should be.

Answer 2

You can't have both at the same time.

If you are not moving, then the static friction is overcoming all other forces and it is in whatever direction is required to negate the net force of all other forces added together into a vector.

If the object is moving, then the kinetic friction will always be opposite the direction of travel and will be of a magnitude given by:
coefficient of friction * mg.

You will have one or the other, but, not both.

Answer 3

They're mutually exclusive.

In some problems you have to find the governing case. Is the applied force large enough to make the mass move (overcome static friction)? Then use sliding or kinetic friction.

If not, then the frictional reaction can only be equal to the net applied force opposite in direction to balance it.

Answer 4

Once you know the difference between static & kinetic friction, you can find your answers. These are key to finding force of friction. Look them up and you will realize what you are missing.