Overview
This lecture explains how to identify and represent different types of forces using free body diagrams, with several practical examples involving blocks, tables, strings, and shelves.
Free Body Diagrams Basics
- Free body diagrams show only the forces acting on a single object.
- Forces are represented as arrows pointing in the direction they act, originating from the object.
Types of Forces in Examples
- A stationary block on a table experiences gravity (weight) downward and a normal force upward from the table.
- The force of gravity (weight) is a long-range force always present due to Earth's mass.
- The normal force is always perpendicular to the contact surface, opposing gravity for stationary objects.
Hanging Block Scenario
- A block hanging from a string has gravity acting downward and tension in the string acting upward.
- Tension (T) balances gravity, keeping the block stationary.
Block Pulled by a Rope
- A block on the ground pulled by a rope (tension to the right) has gravity and normal force vertically.
- Pulling force (tension) is counteracted by friction in the horizontal direction.
- Friction always opposes the direction of motion or attempted motion.
Shelf and Object Example
- For a 5N object resting on a shelf, gravity acts downward and normal force from the shelf acts upward.
- For a 10N shelf with a 5N object, gravity acts downward (10N), and the object's weight creates an additional downward normal force (5N) on the shelf.
- The shelf is held up by two wires; their tensions add to balance the total downward forces (T₁ + T₂ = 15N).
Key Terms & Definitions
- Free Body Diagram — a diagram showing all forces acting on a single object.
- Weight (Force of Gravity) — the force pulling objects toward Earth, calculated as mass × gravity (Fg or W).
- Normal Force — the support force exerted perpendicularly by a surface on an object resting on it.
- Tension — the pulling force transmitted by a string, rope, or cable.
- Friction — the force resisting motion or attempted motion between two surfaces in contact.
Action Items / Next Steps
- Practice drawing free body diagrams for various scenarios.
- Ensure understanding of how forces balance for stationary objects.