Free-body diagrams and interaction pairs
≈ 48 minFree-body diagrams and interaction pairs
A free-body diagram isolates one system and draws every external force acting on it. Common forces include weight toward Earth's centre, normal force perpendicular to a surface, friction opposing relative motion or attempted motion, tension along a rope, and applied forces. Do not draw forces the object exerts on other objects; those belong on another free-body diagram. Newton's third-law pairs are equal and opposite but act on two different systems, while forces in equilibrium act on the same system and can sum to zero. Labelling arrows with force names and directions gives a reader enough evidence to test your reasoning.
Work it through
A book rests on a horizontal table. For the book as the system, weight acts downward and the table's normal force acts upward. They are equal in magnitude if vertical acceleration is zero, but they are not a third-law pair. The third-law partner to the book's weight is the gravitational force the book exerts on Earth; the partner to the normal force is the force the book exerts on the table.
Mastery target
Draw a correct free-body diagram, distinguish equilibrium forces from third-law pairs, and use the diagram to set up a net-force equation.
Which forces act on a book resting on a horizontal table?
Name the key physics term from Free-body diagrams and interaction pairs that best fits the explanation and visual model.
A force of 15 N acts right and 11 N acts left on one object. What is the net force in N, taking right as positive?
Which statement corrects a common misunderstanding in Free-body diagrams and interaction pairs?