Strength is related to movement.
Imagine a block of wood resting on a table. This block tends to remain at rest unless "something" sets it in motion.
This “something” is the action of a force. Imagine that the block is set in motion with the momentary application of a horizontal force. The block moves for a certain space and stops again.
If the block and table are sanded until smooth, applying the same force will allow it to move for a larger space before stopping.
If finally lubricating oil is passed over the table, then the block will slide even further.
This evidence gives us an indication of why the block stops. It stops thanks to its interaction between its surface and the table surface, which comes from the roughness of the two surfaces that scrape together as the block moves. This interaction is friction.. When surfaces are sanded and lubricated, friction is not completely eliminated but is greatly reduced. Another factor that opposes block movement is the air resistance.
And if it were possible to completely eliminate air resistance and friction, what would happen to the block once it was set in motion?
The answer to this question by Isaac Newton In the seventeenth century, which is still considered valid by scientists today, is that the body would remain in rectilinear motion (and straight line) and uniform (with constant speed), forever.
Did you find this strange?
The situation of perpetual motion does not seem obvious to us because we live on a planet where there are at least two factors that make movement analysis difficult: air resistance and friction.
Imagine that it was possible for an astronaut in distant space to be far from any celestial body. If he applied force to a ball to set it in motion, the ball, free of air resistance and friction with other bodies, would continue in uniform rectilinear motion forever.
Newton's first law, or principle of inertia, can be stated as:
|A body that is at rest tends to remain at rest unless a resulting force acts on it. And a body that is in straight and uniform motion tends to remain in even straight motion unless a resulting force acts upon it.|
That the state of rest is a natural tendency and that it is altered only by the application of a force is a relatively easy statement to accept, as it is in agreement with many everyday observations. The great innovation of Newton's first law is to regard rectilinear and uniform motion as a state equivalent to rest, and to assert, therefore, that this state can also be altered only by the action of a resulting force.
A body always offers resistance to alteration in its state of rest or uniform and rectilinear motion. This resistance is called inertia and is a feature of bodies. Each body has its own inertia, and as we will see later, the mass of the body is the measure of that body's inertia.
To illustrate: When a car starts, the bodies inside it are pulled back. When braking, projected forward.