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Inertia and Newton's Laws

We can state Newton's Laws of motion simply as:

A body remains at rest or in a state of uniform motion unless acted on by a resultant force
Force = Mass x Acceleration
Action and reaction are equal and opposite.

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.

Newton's First Law

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.

A body remains at rest or in a state of uniform motion unless acted on by a resultant force

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!


© Keith Gibbs 2020