Forces and Motion | Electricity and Magnetism

More careful thinking about mass

Physics Narrative for 11-14 Supporting Physics Teaching

The idea of mass is subtle: read more about it.

The definition of mass in terms of the amount of stuff is the one which is commonly used in school science. We suggest you do not follow this practice, as the amount of stuff or matter should be measured in moles.

For a physicist:

The mass of an object is a measure of its resistance to being accelerated by a given force.

Thus a 1 kilogram mass has a certain resistance to being accelerated by a given force, and a 2 kilogram mass has double that resistance.

We sometimes express this in terms of inertia, stating that a bigger mass has a greater inertia or reluctance to being accelerated. You might consider the inertia of a juggernaut lorry or a super-tanker ship. Both of these require huge forces to set them moving (to accelerate them from rest). Equally, both need huge forces to bring them to a halt once they are moving (to decelerate them). Both have a large mass and a large inertia.

The relationship between mass and resistance to acceleration can be seen in Newton's Second Law of motion:

acceleration = forcemass

You could also write, less helpfully;

force = mass × acceleration

This equation tells us that an object with a big mass will undergo a small acceleration if a given force is applied to it and vice versa.

So, although the physics definition of mass is in terms of resistance to acceleration, we can see that this measure is directly linked to the amount of stuff idea, in that the more matter or stuff there is in an object, the harder it is to set into motion, or to stop.


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