Force and Free Body Diagrams

Overview

This lecture introduces force diagrams and free body diagrams, explaining the forces acting on objects such as a box at rest or in motion on a table.

Forces Acting on a Box

• A box on a table experiences a downward gravitational force (weight).
• The table provides an upward normal force to support the box.
• If only gravity acted, the box would fall; the normal force prevents this.

Force Diagrams vs. Free Body Diagrams

• A force diagram shows the actual object with force arrows labeled from its center.
• A free body diagram uses a simple dot; all force arrows point away from the dot representing the object.
• In both diagrams, label all forces and add a key for clarity.
• Only the object of interest (not the table or surroundings) is drawn in these diagrams.

Forces When a Box Is Pushed

• An applied (pushing) force is exerted on the box.
• Friction opposes the motion due to microscopic imperfections between the box and the surface.
• Gravity acts downwards; the normal force acts upwards from the surface.

Drawing Force and Free Body Diagrams

• In force diagrams, draw and label all forces acting on the object.
• In free body diagrams, represent the object as a dot with arrows pointing away for each force.
• Free body diagrams are simpler and more commonly used in physics problems.

Key Terms & Definitions

• Force of Gravity (F₉ or W) — Downward force on an object due to Earth's gravity, also called weight.
• Normal Force (N) — Upward support force from a surface that prevents an object from falling through.
• Friction (fₓₓ or FF) — Force opposing motion due to microscopic surface irregularities.
• Applied Force (Fₐ) — Force exerted on an object by a person or another object.
• Force Diagram — Shows the object with labeled arrows for each force.
• Free Body Diagram — Simplified diagram with a dot and arrows for all acting forces.

Action Items / Next Steps

• Practice drawing force diagrams and free body diagrams for different scenarios.
• Memorize definitions for key physics terms: gravity, normal force, friction, and applied force.