Mass, inertia and Newton's first law

By the end of this lesson

By the end you can say what mass really measures (how strongly an object opposes a change in its motion), and state and apply Newton's first law.

The tablecloth trick

Yank a tablecloth out from under a full dinner set fast enough and the plates barely move. Not magic: mass.

Every object resists having its motion changed. The cloth only drags on the plates for a split second, and that brief, weak tug can't get them moving much. (Pull too slowly and you'll need a dustpan.)

Definition

Mass

Mass is the property of an object that opposes (resists) any change in its motion. The bigger the mass, the harder it is to speed the object up, slow it down or change its direction.

This resistance to change is called inertia. Mass is a scalar and an SI base quantity, measured in kilograms (kg).

Mass is stubbornness

Push an empty shopping trolley and one loaded with 40 bottles of water equally hard. The full one speeds up far less, and it's just as hard to stop once it's rolling.
On the International Space Station everything floats, yet a big storage crate still takes a hefty shove to start and another to stop. No weight is involved there: that's pure mass.
The station even measures its astronauts' masses this way: a device pulls them with a known force, measures their acceleration and works out the mass.

Where the analogy breaks: "stubborn" sounds as if the object pushes back with a force of its own. It doesn't. Mass is a property of the object, not a force.

Quick check

Fill in the blanksQuestion 1 · Recall

Multiple choiceQuestion 2 · Apply

Answer the 2 questions above to continue.