How do you find the "Force Applied" in a problem about Kinetic Friction??

Answer 1

In general:

Construct a free-body diagram for the object under consideration.

Break the forces into components along a coordinate system. It is almost always best to choose a coordinate system where one axis is in the direction of motion, if any.

Apply Newton's Second Law to each dimension:

The sum of the forces = mass times acceleration.

If there's no acceleration in that dimension (typically the vertical dimension) then the sum of the forces is zero. This is often how you find the magnitude of the so-called "Normal Force."

The force of friction is usually expressed as uN, where u is the coefficient of friction and N the normal force that you just found.