Forces and Newton's Laws: Conceptual Physics Study Guide

Newton's First Law (Law of Inertia)

Objects keep doing what they're already doing (rest or constant velocity) unless a net external force acts on them.

Inertia

Depends on mass — more mass = more inertia.

Newton's Second Law

Describes how forces cause acceleration.

Net Force

Relationship between net force, mass, and acceleration.

Newton's Third Law

Forces always come in pairs, equal in magnitude and opposite in direction.

Major Types of Forces

Gravity (weight), normal force, friction, tension, applied forces, air resistance/drag, spring force.

Contact Forces

Require physical contact.

Field Forces

Act at a distance (gravity, magnetism, electric forces).

Free-Body Diagrams (FBDs)

Show all forces on a single object.

Balanced Forces

Net force = 0; acceleration = 0.

Unbalanced Forces

Net force ≠ 0; object accelerates.

Static Friction

Prevents motion; adjusts up to a maximum limit.

Kinetic Friction

Acts when surfaces are sliding; generally smaller than static friction.

Factors That Affect Friction

Coefficient of friction and normal force.

Coefficients of Friction

Dimensionless numbers (no units); μ > μₖ for most surfaces.

Common Misconceptions About Forces

Motion does NOT require a continual force; heavy objects do not fall slower due to weaker gravity.

Air Resistance

Acts opposite to motion through air; increases with speed.

Terminal Velocity

When forces balance (weight = drag); no acceleration.

Apparent Weight

What a scale reads; changes if the elevator accelerates.

Action-Reaction Force Pairs

Occur between two interacting objects; equal in magnitude and opposite in direction.

Friction

Depends on surface roughness and force pressing surfaces together.

Drag

Changes with speed.

Free Fall

Apparent weight = 0.

Tension

The upward force in a rope supporting a hanging object.

Apparent Weight in Elevators

Changes based on elevator's acceleration.