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# Students’ Common Mistakes in Making Free-Body Diagrams

Students find free-body diagrams confusing. While it is easy to read any such diagram given, making one from the given description in a question can be problematic. Here we discuss the common mistakes in making a free-body diagram.

A typical free-body diagram showing a body on a slope surface. The three arrows indicate the forces acting on the body, namely weight (F1), Normal reaction force (F2), and friction (F3).

Picture taken from http://ffden-2.phys.uaf.edu/211_fall2004.web.dir/Jeff_Levison/Freebody%20diagram.htm

## Mistake 1: Missing Weight and Normal Reaction Force

The question may mention nothing about gravity and the normal reaction force acting on a body. Students need to aware of these forces and include them in the diagram.

(a) Example

(b) Wrong Diagram

(c) Correction

(a) Example: A person is pushing a box. The floor exerts friction on the box. (b) Wrong diagram: The weight and normal reaction force are missing. (c) Correction: With the inclusion of weight and normal reaction force, all the forces are present and the free-body diagram is considered complete.

## Mistake 2: “Centripetal Force” was Added to the Free-Body Diagram

Centripetal force is a tricky item when it comes to free-body diagram. The short answer is – do not add centripetal force in a free-body diagram because it is not one of those contributing forces like gravity, normal reaction force, tension force, etc. It is simply a convenient way of describing the effect o resultant force.

(a) Example

(b) Wrong Diagram

(c) Correction

(a) Example: A ball attached to a string is performing circular motion in vertical plane. (b) Wrong diagram: The “centripetal force” is added in addition to the weight and string tension. (c) Correction: The “centripetal force” is the result rather than the contributing forces. Hence, do not add it in the free-body diagram.

## Mistake 3: The Force is not Acting on this Body

There could be multiple bodies in the question. It is important to figure out the individual forces acting on each body. A force acting on body #1 should not appear in the free-body diagram for body #2.

Example

Correct Diagram

(a) Example: A ball attached to a string is performing circular motion in vertical plane. (b) Wrong diagram: The “centripetal force (CF)” is added in addition to the weight and string tension. The “centripetal force (CF)” is the result rather than the contributing forces. (c) Correction: Hence, do not add it in the free-body diagram.