Newton's Laws of Motion

Overview

The lecture introduces Newton’s three laws of motion, focusing on how forces cause acceleration, the concept of inertia, and key force types like gravity, normal force, and tension, with real-world examples and essential problem-solving steps.

Newton’s First Law: Inertia

• Objects resist changes in motion; this property is called inertia.
• An object at rest stays at rest, and an object in motion stays in motion unless acted upon by a net force.
• Mass measures inertia; more mass means more resistance to changes in motion.

Newton’s Second Law: Force and Acceleration

• Net force equals mass times acceleration, summarized as F(net) = ma.
• Net force is the sum of all forces acting on an object after accounting for those that cancel each other.
• If the net force on an object is zero, its velocity remains constant (equilibrium).

Common Example: Gravity and Weight

• Gravity pulls objects downward at 9.81 m/s² (“small g”).
• The force of gravity (weight) is calculated as F(g) = mg.
• Weight is measured in Newtons (N), not kilograms; kilograms measure mass.

Newton’s Third Law: Action and Reaction

• For every action, there is an equal and opposite reaction.
• Normal force is the upward, perpendicular reaction force from a surface.
• The magnitude of the normal force adjusts to match the downward force, up to a breaking point.
• Action-reaction pairs allow for movement when other forces (like friction) are present.

Forces in Equilibrium & Free Body Diagrams

• A free body diagram (FBD) visually represents all the forces acting on an object.
• For objects at rest or moving at constant velocity, upward and downward forces are equal; net force is zero.
• Tension is the pulling force within ropes or cables; it matches the force needed to hold the object.

Example: Elevator and Counterweight System

• For an elevator system, draw FBDs to analyze forces on both the lift and the counterweight.
• Set up equations using F(net) = ma for each object.
• Use algebra to eliminate unknowns (like tension) and solve for acceleration.
• The acceleration of the system is the net force divided by the total mass.

Key Terms & Definitions

• Inertia — An object’s resistance to change in its state of motion.
• Net Force — The overall force acting on an object after all forces are combined.
• Equilibrium — A state where the net force on an object is zero; velocity is constant.
• Normal Force — The perpendicular reaction force from a surface supporting an object.
• Tension — The pulling force transmitted by a string, rope, or cable.
• Weight — The gravitational force on an object; calculated as mass × gravitational acceleration (mg).
• Free Body Diagram (FBD) — A diagram showing all forces acting on a single object.

Action Items / Next Steps

• Practice drawing free body diagrams for different scenarios.
• Use F(net) = ma to solve for unknowns in force problems.
• Review the definitions of force types and Newton’s laws.