Force Identification Errors
Incorrect Thinking
Students may draw velocity, acceleration, or “motion force” as forces, while omitting contact forces such as normal reaction, tension, friction, or drag.
Why Students Think This
Motion arrows and force arrows are both drawn as vectors, so students may blur the distinction between a kinematic quantity and an interaction.
Correction
Only interactions are forces. Start each free-body diagram by identifying the object of interest and listing external interactions with other bodies or fields.