Weight and Mass
In case you didn’t think there was any difference...
Mass
What it measures:
Basically, the amount of stuff (as opposed to
volume, which is the amount of space the
stuff takes up).
Weight
What it measures:
The force that gravity exerts on anything
with mass – the more the mass, the greater
the weight.
What it’s measured in:
SI units are kilograms (
), although it can
also be measured in grams, tonnes, etc, or
even in imperial units.
What it’s measured in:
SI units are Newtons (
) (it is a force, after
all), although using we can see that
this is equivalent to
.
How it can be experienced:
Through inertia/momentum (it takes a lot
more work to get something moving if it has
a large mass), or through the effect it has on
weight (see opposite).
How it can be experienced:
Only indirectly – generally what we think of
as our weight is actually the normal reaction
forces that act on us as a result of our
weight. A feeling of ‘weightlessness’ is
essentially a feeling of no forces working
against the weight (ie, in freefall).
Food for thought:
Imagine rugby tackling someone on the
moon – the impact of tackling someone
much bigger (more ‘massive’) than you
would be pretty much the same as on Earth.
Because you’re tackling horizontally, you’re
not having to work against gravity either on
Earth or on the Moon, just inertia.
Food for thought:
When going up in a lift, you feel heavy at first
not because your weight has changed but
because the normal reaction acting on you
from the floor of the lift has increased (to
accelerate you upwards). And you feel
lighter as the lift decelerates because your
normal reaction is then lower than usual.
On Earth, your weight is directly proportional to your mass (always ), but on the Moon,
since the acceleration due to gravity is lower, while weight is still directly proportional to
mass, the constant of proportionality is lower (so we get about ).
Recall that the acceleration due to gravity is the same for any object (close to the surface of
the Earth), and is written as
. This means any object, neglecting air resistance,
should fall at the same rate.
