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A free-body diagram usually depicts a single object, and in an introductory physics class, this object is usually a familiar thing composed entirely of matter in a solid phase. Light or liquids are not typically depicted in a free-body diagram. The object is sometimes shown in a two or three-dimensional representation, but this is usually unnecessary because the effects of torque (twisting) are usually not considered in introductory physics either. In fact, a free-body diagram can be most effectively drawn as a zero-dimensional point. Essentially, the three-dimensional object is simplified down to this zero-dimensional dot. Its environment is ignored. Instead, all of the forces acting on this object, now represented as a dot, are drawn, typically as arrows, either pointing to or away from the dot.
The forces, usually shown as vectors, can be used to trigonometrically calculate the net force acting on the body, and therefore its subsequent acceleration if the diagram were allowed to become "unfrozen" and play out the effects of the forces on the object.
The most common forces depicted in a free body diagram are gravity, the normal force, friction, and some form of thrust. A free-body diagram for an box being pushed across a table might look like this:
Box: a dot in the center of the diagram with its mass labelled
Gravity: an arrow pointing straight down, with force equal to mg.
Normal force: an arrow pointing straight up, with force equal to mg. Another way to think of the normal force is that it counteracts the weight of the box due to gravity and prevents it from falling through the table.
Thrust: an arrow pointing in the direction of the box's motion, with force equal to whatever is given in the problem.
Friction: an arrow pointing in the opposite direction of the box's motion, with force equal to the coefficient of friction (a measure of the resistance between the box and the table) and the normal force.
The link below shows what this diagram might look like.
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