Dynamics · Physics

Weight vs Mass

Mass is an intrinsic property. Weight is a force that depends on gravity and motion.

This topic includes

Subtopics to master

Get the definitions right, then master apparent weight and “scale reading” problems.

Definition
Definition of Mass
Mass measures inertia and sets how much acceleration results from a given net force.
  • Inertial vs gravitational mass
  • Units (kg)
  • Constant across locations
Start subtopicMass ≠ weight.
Definition
Definition of Weight
Weight is the gravitational force on an object (near Earth often W = mg).
  • Force units (N)
  • Direction: toward Earth’s center
  • Varies with g
Start subtopicForce concept.
Key
Weight as a Force
Weight belongs on FBDs. Mass belongs in ΣF = ma.
  • Place mg on the diagram
  • Don’t put “m” as a force
  • Common representation errors
Start subtopicDiagram correctness.
Scales
Apparent Weight
Apparent weight is often what a scale reads: typically the normal force, not mg.
  • Elevator problems
  • Up/down acceleration cases
  • When you feel “heavier”
Start subtopicClassic application.
Environment
Mass vs Weight in Different Environments
Mass stays the same; weight changes with local gravity and motion state.
  • Moon vs Earth
  • Spacecraft “weightlessness”
  • g variations (intro)
Start subtopicBuild intuition.
Practice
Practice & Exercises
Definition checks and scale-reading problems with acceleration.
  • Mass vs weight sorting
  • Convert kg ↔ N (using g)
  • Elevator apparent-weight drills
  • FBD + ΣF equation sets
  • Short explanation prompts
Open practiceEliminate confusion.