Force Diagrams

Learn the models, terms, and applications of force diagrams.

Lesson 1 of 32 parts Practice Flashcards

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Free-Body Diagrams

Objective: Draw and interpret forces acting on one chosen object.

A free-body diagram isolates one target object and shows external forces acting on it as labeled arrows. Arrow directions represent force directions; lengths can indicate relative magnitudes when a scale is specified. Do not draw forces that the object exerts on other objects on the same diagram. A book resting on a table generally has a downward gravitational force and an upward normal force; if stationary in an inertial frame, those vectors sum to zero. The normal force is a contact force perpendicular to the surface and is not automatically equal to weight in every situation. A velocity arrow is not a force arrow.

Worked examples

Try itA book rests on a level table. Which forces act on the book?Show answer

1. Identify the relevant concept in this section. 2. Draw forces applied to the book. 3. Answer: Earth’s gravity down and table’s normal force up

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Free-Body Diagrams

Objective: Draw and interpret forces acting on one chosen object.

Worked examples

Try itA book rests on a level table. Which forces act on the book?Show answer

1. Identify the relevant concept in this section. 2. Draw forces applied to the book. 3. Answer: Earth’s gravity down and table’s normal force up

Applications and Worked Problems

Objective: Apply force diagrams to calculations, evidence, and specified situations.

A free-body diagram includes forces acting on one selected object, not forces it exerts on other bodies. Add force components with signs to obtain the net force. Mass and velocity are not extra force arrows. Weight near Earth is mg; a normal force is a contact force perpendicular to the surface and does not always equal weight. At rest or constant velocity in an inertial frame, net force is zero. Third-law partners act on different objects and therefore do not cancel within one object’s diagram.

Formulas

$F_net=ΣF$
weight≈mg near Earth

Worked examples

Try itA cart experiences 15 N right and 4 N left. Take right positive. Find net horizontal force.Show answer

1. Add forces as signed components. 2. 15−4=11 N.

Try itA stationary hanging object of mass 3 kg has no other vertical force. Find supporting tension with g=9.8 m/s².Show answer

1. At rest, vertical net force is zero. 2. T=mg=3×9.8=29.4 N.

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