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.