Galileo and Inertia

Inertia is the tendency of matter to resist changes in its velocity.

Inertia is a property of matter. It is that property of matter which opposes changes in velocity.

Simply stated, a common object will not change its velocity spontaneously.

If something is moving along at a constant speed in a straight line, it will continue to move along at the same constant speed in the same straight line. It will not, all on its own, speed up, slow down, or change direction. Something else must push on the object to speed it up, slow it down, or change its direction.

Also, if something is standing still, it will, if left to itself, continue to stand still. Something else must push on an object to get it moving. Actually, an object standing still is just a special case of an object keeping its velocity constant. Its velocity is constantly 0 m/s.


Objects do not spontaneously change their velocities.

Center your thoughts:

An object will not change its velocity on its own.

Pushes, or pulls, from the outside are necessary to change an object's velocity.


Pushes, or pulls, are not necessary to keep a stationary object still.

Once at rest, an object will stay at rest all on its own.

Pushes, or pulls, are not necessary to keep an object moving.

An object will keep moving all on its own.


Usually, Galileo gets the credit.

Again, the property of matter that is responsible for this nature is called inertia. Galileo is traditionally credited with being the first scientist to formalize this concept. People before him often had it turned around. Many believed that a push was necessary to keep something moving. It certainly seems that way at first thought. If you push a chair across a room, it seems that your push is necessary to sustain the velocity of the chair. If you stop pushing, the chair stops moving.

Galileo, though, believed that when the push on the chair is taken away, the chair should continue to move along without any assistance. And, as it turns out, it will, if the chair is entirely left alone. By 'left alone' we mean that nothing pushes or pulls on the chair.

But the chair is not left alone. There is a force of friction between the chair and the floor. Friction continues to apply a push to the chair after you take your hand away from it. It is this friction that prevents the chair from continuing its motion. The friction quickly brings the chair to a halt. Without friction, the chair would just keep moving.

So, if you get something moving, and then leave it alone, it will continue to move without any more pushes from you. Again, by leaving it alone we mean that no pushes or pulls are placed upon it.


As high as the ball rolls on the left, it rolls on the right.

Galileo figured this out by thinking of a ball rolling down an incline and and then up an identical incline. He imagined this motion:

We must think of a very smooth ball and very smooth inclines. So smooth, in fact, that we do not have to worry about friction slowing down the ball. Also, do not worry about the ball bouncing about when it abruptly changes direction at the bottom of each incline. Imagine this transaction to be gradual, much like a marble rolling in a large bowl.

Galileo noticed that the ball could be started at a certain height on the left incline. It rolls over to the right incline and rolls up that incline to the same height from which it was released on the left. Of course, for a real situation, the ball would not make it to the other height exactly due to friction, so Galileo could not have really seen this. But here Galileo and we are reasoning from experience and proceeding through a thought experiment where we imagine what things would be like without outside influences, say from friction.


The ball might roll farther, but just as high.

Then he reasoned what would happen if the right incline was not so steep. That motion looks like this:

The ball again rises to the same height from which it was released. Now, however, the ball must roll a greater distance up the right incline before coming to a stop for an instant at the top of its journey. Therefore, it takes more time for the ball to roll up the right incline than down the left, and the ball has rolled a greater distance on the right incline than on the left..

Be sure to notice that it travels farther on the left incline.


The ball might roll much farther, but just as high.

Then Galileo demonstrated that the ball would roll for an even longer time on the right incline before coming to a stop if that incline was made even less steep. That motion would look like this:

The less the slope on the right, the farther the ball rolls.


The ball will roll forever without the left hill.

Now, Galileo asks a simple question - How long would the ball roll before coming to a stop if you made the right incline flat, that is, if you took away the right incline? He realized it would roll for an infinite amount of time; the ball would not stop rolling. It would continue moving along with an unchanging velocity as long as nothing else effected it. That is, as long as it experienced no other pushes or pulls. The motion without the right incline would look like this:

The ball will, all on its own, continue in its state of motion, moving at a constant speed in a straight line. This property of matter is called inertia.

If you want to change its velocity, you will have to push on it, and it will push back. It will resist with an equal and opposite force. That is what is meant by 'Inertia is that tendency of matter to resist changes in its velocity'.


Objects will not change their velocities unless they are forced to by forces.

That is how Galileo came up with the concept of inertia. Inertia is that property of matter that opposes changes in motion. Or, one might say, inertia is that property of matter that keeps the velocity constant. If an object is in motion, it will continue moving without help from the outside. The velocity of an object will not change unless you push or pull on the object. A push or a pull is called a force.

Forces create changes in velocity as time passes.

When we say that the velocity of an object does not change we mean that its speed does not change and the direction in which it is moving does not change. It continues with unchanging speed in a straight line.

Note that an object standing still maintains a constant velocity. It's velocity is zero. An object with zero constant velocity will continue to have zero constant velocity if you do not apply forces to it. Objects standing still will continue to stand still. They will not start to move on their own unless they receive a push or pull from something.


Isaac Newton's first law of motion captures the concept of inertia.

Throughout this discussion we have used the words push and pull. The term for a push or a pull is force. Therefore, you can only change the velocity of an object if you place a force on the object. This is Sir Isaac Newton's First Law of Motion. Newton's first law of motion. It is basically the same as Galileo's explanation of inertia.


Zero net force works the same as 'left alone'.

Throughout this discussion we have used the term 'left alone' to mean that no forces (no pushes or pulls) are placed on the object. Actually, several forces can act on an object, but if they cancel each other, then the effect is the same as if no forces at all were applied. For example a push to the right could be canceled by an equally strong push to the left. In this case the object has  two forces on it, but it moves as if it had zero force on it. This summing of forces is called calculating the net force. Here's a link to section on net force.


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