FirstPrinciplesa Physoly project
NL-09mechanicstrap · Dimensional inconsistency

Mass and weight are different quantities

  1. 01

    The tempting model

    Using kilograms as a force, or reporting weight in kg.

  2. 02

    Why it feels right

    Everyday scales display a mass estimate in kilograms even though their sensing element responds to force.

  3. 03

    Where it breaks · how to catch it

    A force term carries units of kg, or you added a mass to a force.

  4. 04

    The correct model

    Mass (kg) is inertia; weight (N) is the gravitational force mg. On the Moon your mass is unchanged but your weight is ~1/6.

  5. 05

    Repair · see it, then test it

    worked example A 5 kg bag — what is its weight on Earth?

    Tempting move

    5 kg.

    Correct move

    W = mg = 5·9.8 = 49 N.

    Test the repair NL-09 · transfer
    include N

    Make a fresh prediction before referring to the worked example.

    Then practise it on the ladder

    R1Identify the forces on a blockadds: A free-body diagram lists only real forces from real agents.