Overview
This lecture covers Newton's three laws of motion, the concept of force, types of forces, free body diagrams, friction, spring force, drag force, and equilibrium conditions.
Introduction to Forces
- Force is defined as a push or pull acting on an object.
- Forces are measured in Newtons (N), where 1 N = 1 kg·m/s².
- Two types of forces: contact forces (physical touch) and field forces (no contact, e.g., gravity).
- Force is a vector, requiring both magnitude and direction.
Newton’s Laws of Motion
- First Law (Law of Inertia): An object at rest stays at rest, and an object in motion stays in motion unless acted upon by a net force.
- Second Law: The net force on an object equals mass times acceleration (ΣF = m·a).
- Third Law: For every action, there is an equal and opposite reaction; forces act on different objects.
Mass vs. Weight
- Mass is the property of matter measured in kilograms; it does not change with location.
- Weight is the force on an object due to gravity (Weight = mass × gravitational acceleration, w = m·g).
- Gravitational acceleration (g) varies by celestial body (Earth: 9.80 m/s²).
Common Types of Forces
- Gravity: Acts downward toward the center of the Earth (F = m·g).
- Normal Force: Perpendicular contact force from a surface, directed toward the object.
- Tension: Force along a stretched string, rope, or cable.
- Friction: Opposes motion, acts parallel to surface.
- Spring Force: Proportional to the change in length, F = -k·ΔL (Hooke’s Law).
- Drag Force: Opposes motion due to air resistance, leads to terminal velocity.
Free Body Diagrams
- Diagram showing all forces acting on an object, with arrows representing vectors.
- Useful for analyzing forces and solving problems involving Newton’s laws.
Friction
- Frictional force is proportional to normal force (F_friction = μ·F_normal).
- μ (coefficient of friction) depends on the surfaces in contact.
- Static friction acts when objects are at rest; kinetic friction acts when objects are moving.
- Static friction coefficient is greater than kinetic friction coefficient.
- Orientation of an object does not affect friction if mass and surfaces remain the same.
Spring and Hooke’s Law
- The spring constant (k) is unique to each spring.
- Elastic region: spring obeys Hooke's Law; plastic region: permanent deformation occurs.
- Beyond elastic limit, the spring will not return to original length.
Equilibrium Conditions
- An object is in equilibrium when the net force in all directions (x, y, z) is zero.
- For true equilibrium, net torque must also be zero (no unbalanced rotation).
Key Terms & Definitions
- Force — Push or pull on an object, measured in Newtons (N).
- Contact Force — Force with physical contact.
- Field Force — Force acting over a distance (e.g., gravity).
- Normal Force — Perpendicular force from a surface.
- Friction — Force opposing motion, acts parallel to the surface.
- Static Friction — Friction when objects are at rest.
- Kinetic Friction — Friction when objects are moving.
- Spring Constant (k) — Measure of a spring’s stiffness.
- Equilibrium — State where net force and net torque are zero.
Action Items / Next Steps
- Attend the recitation session.
- Complete the worksheet and assigned homework.