Overview
This lecture introduces fundamental types of forces in mechanics, focusing on contact and non-contact forces, their correct symbols, and how they are represented and analyzed in physics problems.
Types of Forces
- Forces are classified as contact (e.g., normal, friction, tension, applied) or non-contact (e.g., gravitational/weight).
- In mechanics and Newton's Laws, most forces considered are contact forces, with gravitational force as a key non-contact force.
- Common forces and symbols include: normal (FN or N), applied (F_applied), tension (T), friction (F_f, F_s for static, F_k for kinetic), weight (W or F_g).
- Electrostatic and magnetic forces are non-contact forces but covered in other chapters.
Force Representation and Diagrams
- Correct symbols are essential for free body diagrams and scoring marks in exams.
- Free body diagrams show all forces acting on an object, with arrows pointing away from the object.
- In diagrams, forces are represented as arrows in the direction they act (e.g., push or pull).
Applied Force
- Applied force is any push or pull that may cause acceleration or constant velocity.
- It can act at any angle, requiring resolution into horizontal (x) and vertical (y) components for analysis.
- Force diagrams for applied force must show the correct direction and don't include components when drawing for marks.
Tension Force
- Tension is the force within a rope or string when it is stretched or connects objects.
- Tension acts along the rope, pulling the connected objects in opposite directions.
Forces Exerted by Surfaces
- Normal force acts perpendicular (90°) to the surface, supporting the object's weight.
- Frictional force acts parallel to the surface, always opposing the direction of motion.
- Friction remains parallel regardless of the direction of the applied force or if the surface is inclined.
Gravitational Force (Weight)
- Weight is the gravitational force exerted by the Earth on an object near its surface.
- The direction of weight is always straight down to the Earth's center, not perpendicular to surfaces.
- Formula: Weight = mass × gravitational acceleration (g = 9.8 m/s² on Earth).
Key Terms & Definitions
- Contact Force — Force requiring physical contact between objects.
- Non-contact Force — Force acting at a distance (e.g., gravity).
- Normal Force (FN or N) — Perpendicular support force exerted by a surface on an object.
- Applied Force (F_applied) — External push or pull applied to an object.
- Tension (T) — Force transmitted through a string, rope, or cable.
- Friction (F_f, F_s, F_k) — Force opposing motion, parallel to the surface.
- Static Friction (F_s) — Friction preventing motion between two surfaces.
- Kinetic Friction (F_k) — Friction opposing motion between moving surfaces.
- Weight (W or F_g) — Force of gravity acting on an object’s mass.
Action Items / Next Steps
- Review upcoming lectures on calculating normal force, weight, and friction.
- Practice drawing correct free body diagrams using proper force symbols.
- Prepare to resolve forces into components for Newton's Laws questions.