← Weight, mass and free-body diagrams

Weight or mass? Judge the claim

A moving company's paperwork reads: 'Sofa, weight 40 kg.' A physics student crosses the line out. The mover shrugs: everyone says it that way. Who is right, and what exactly is wrong?

Weight and mass each have a unit and a behavior. A claim that mixes them up can be caught two ways.

Check one is the unit check. Mass is stated in kilograms. Weight is a force, stated in newtons.

Check two is the location check. Mass never changes with location. Weight does.

CheckMassWeightunitkilograms (kg)newtons (N)locationnever changes with itchanges with it
The unit check and the location check.

Now judge claims with the two checks.

'My suitcase's mass is 23 kg.' A mass stated in kilograms passes the unit check. Correct.

'My suitcase's weight is 23 kg.' A weight stated in kilograms fails the unit check. Incorrect. The 23 kg is the suitcase's mass.

ClaimWhich check decidesVerdict'Mass: 23 kg.'unit: mass in kilogramscorrect'Weight: 23 kg.'unit: weight needs newtonsincorrect
The same 23 kg, two claims about one suitcase. Only the mass claim survives the unit check.

Mira says: 'My bike's mass is 11 kg.' Is Mira using the word correctly?

Jax says: 'My skateboard's weight is 3 kg.' Is Jax using the word correctly?

'The astronaut's weight is smaller on the Moon.' Weight changing with location passes the location check. Correct.

'The astronaut's mass is smaller on the Moon.' Mass changing with location fails the location check. Incorrect. Mass is the same everywhere.

ClaimWhich check decidesVerdict'Weight: smaller on the Moon.'location: weight changescorrect'Mass: smaller on the Moon.'location: mass never changesincorrect
The same trip, two claims about one astronaut. Only the weight claim survives the location check.

Mars's gravity is weaker than Earth's. Tobias says: 'A rover's weight on Mars is less than its weight on Earth.' Is Tobias correct?

One more claim. 'A full water bottle has more mass than the same bottle empty.' More matter means more mass, and the claim has no unit trouble and no location trouble. Correct.

So who was right about the sofa? Run the unit check: a weight cannot be 40 kg. The 40 kg on the paperwork is the sofa's mass. The student was right.

To judge any weight-or-mass claim, run the unit check, then the location check. A claim that fails either check is using the words incorrectly.

A shipping label reads: 'Contents: mass 12 N.' Judge the label.

Run the unit check: mass is stated in kilograms, and the newton is the force unit.

Incorrect. A mass cannot be 12 N. In newtons, 12 N could only be a force, such as a weight.

A quiz answer reads: 'Your mass on Mars is less than on Earth, because Mars pulls more weakly.' Judge the answer.

Run the location check: mass never changes with location.

The weaker pull changes the weight, not the mass.

Incorrect. The quantity that is smaller on Mars is the weight. The mass is the same there.

Your turn

Which statement uses mass correctly?

A gym sign reads: 'This dumbbell weighs 10 kg.' In this course's terms, what is wrong with the sign?

A space agency checklist describes one space suit four ways. Which description is correct?