Caption: Two free body diagrams showing the forces acting on an object (which in this case is a block):

1. The object is on a level plane (a zero-inclination inclined plane: i.e., inclination angle θ = 0°),
2. The object is on an inclined plane (or wedge) with inclination angle θ.

Features:

1. Free body diagrams are stylized diagrams used to analyze forces on objects (often neglecting internal structure).

2. Emphasis: The forces that act on an object are being analyzed and labeled, NOT the forces the object exerts on its environment according to Newton's 3rd law of motion (i.e., reaction forces). If you try to include the reaction forces, you can create confusion for yourself with seemingly totally canceling forces everywhere.

3. In the shown free body diagrams, the forces are:
   1. Gravity W = (mass of object) x gravitational field = mg downward. Note gravity is a body force that acts on every bit of the object. Note also Earth's surface gravitational field g = 9.8 N/kg (fiducial value).

   2. Normal force N = mgcos(θ) perpendicular to the contact surface of the object. and acting on the contact surface of the object. Recall θ is the inclination angle. This force law is derived from Newton's 3rd law of motion with the constraint of a rigid surface: it is a constraint force.

   3. Kinetic friction F = kinetic friction coefficient x normal force = μ_k*N = μ_k*mgcos(θ) parallel to the inclined plane opposite to the direction of motion and acting at the contact surface of the object. The force law for kinetic friction is an approximate physical law that is highly accurate in many cases. It sometimes called Amontons' law.

      Note Amontons' law only applies if the object is in motion. If the object is at rest, the law is

         static friction force = (opposite applied force of whatever kind) ≤ μ_s*mg

      (which is approximate too, but often highly accurate), where μ_s is the static friction coefficient. Note that with static friction, the object can be at rest, but with NO static friction, the object must accelerate down the inclined plane for θ > 0°. In a physics description, the 3 forces can add as vectors to zero, but without friction the 2 remaining forces CANNOT.

      The friction coefficients are functions of the contact materials. Both friction laws are for dry friction.

Credit/Permission: User:Penubag 2007 / Public domain.
Image link: Wikipedia: File:Freebodydiagram3 pn.svg.
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Extended File: Mechanics file: free_body_diagram_object_wedge_4.html.
File: Mechanics file: free_body_diagram_object_wedge.html.