Also we define inertia as:
The
inertia of a body is its reluctance to change its state of motion
The more massive
the body is the more inertia it has. The astronauts in the Space Shuttle had trouble with large
pieces of equipment since although they were "weightless" they still had inertia and were
difficult to stop once they were moving. The astronauts themselves also experienced a
problem on the lunar surface. They had a much smaller weight and therefore lower friction
between themselves and the surface but their mass and therefore inertia was the same as
on Earth. Stopping was difficult due to the lower frictional force.
The period of
vibration of a body is also affected by its inertia. Large, heavy objects will vibrate slowly. One
simple way of testing this is to use a ruler loaded with a lump of plasticine - the greater the
load the longer the period of the motion. Therefore the rate of vibration can be used to
compare the masses of two objects since the rate of vibration depends on inertia (mass) and
not weight.
Aristotle thought that a force was needed to keep an object moving and
that if this force was removed the object would naturally come to rest. Galileo - not being
totally happy with this idea thought about motion in terms of what might happen to a ball
rolling down one side of a U shaped slope. He reasoned that if there was no friction the ball
would go down one side and then up the other until it reached the same height as that from
which it started (think about this in the design of big dippers). Now if the other side of the U is
steadily lowered the ball will have to go further before it reaches its original height.
Logically
if the slope is flattened out the ball will roll on for every - never getting to its original height.
So no force is needed to keep it going.
Newton expressed this idea in his first law of
motion.
Think about
the law in two parts:
(a) at rest
The word resultant really ought to go in
because clearly someone sitting on a stool may be at rest but they are acted on by two
forces - their weight and the reaction of the stool. It is because these two forces are
balanced and there is no resultant force they stay still - i.e. at rest.
(b) uniform
motion
this means no change of velocity - and since velocity is a vector this means at a
steady speed in a straight line. Think about a sky diver, as they fall out of the plane their
speed increases - their weight is bigger than the drag - there is a net force and Newton's first
law does not apply. However as the drag increases the two forces on them become equal
and the sky diver falls with a constant velocity - a state of uniform motion!