Laziness ≈ Inertia

For centuries physics slept in Aristotle’s (384-322 BC) shadow.

Aristotle believed that the “natural” motion of celestial objects was circular,

while terrestrial objects tend “naturally” to fall.

Notice if the moon naturally moves in a circle, we don’t need any gravity to explain its motion.

As for earthly objects, Aristotle believed that after falling, they come to rest, unless some force pushes them.

It does seem that a force is needed to maintain motion.

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.

But it is not left alone.

Friction between the chair and the floor 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.

Galileo figured this out by thinking of a ball rolling back and forth between two identical inclines.

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.

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 incline.

Then he reasoned what would happen if the right incline was not so steep.

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.

Now, Galileo asks a simple question - How long would the ball roll before coming to a stop if you made the right incline flat?

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 affected it.

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.

The reluctance of an object to change its state of

motion is INERTIA.

The amount of inertia an object has depends on its mass.

The more mass an object has, the greater its inertia AND the greater the force required to change the object’s state of motion.

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.

That is, 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.

Galileo is traditionally credited with being the first scientist to formalize this concept.

Isaac Newton (1642-1727) summarized Galileo’s idea as Newton’s First Law:

Every object continues in a state of rest, or of motion in a straight line at

a constant speed, unless it is compelled to change that state by unbalanced forces exerted upon it.

A. Describe how the picture below demonstrates the law of inertia.

B. A girl attaches a rock to a string, which she then swings counterclockwise in a horizontal circle. The string breaks at point P on the sketch, which shows a bird's-eye view (i.e., as seen from above). What path will the rock follow?

C. A 4.0 kg object is moving across a frictionless surface with a constant velocity of 2 m/s. Which one of the following horizontal forces is necessary to maintain this state of motion?

 A. 0 NB. 0.5 NC. 2.0 ND. depends on the speed

But what exactly is meant by the phrase unbalanced force?

What is an unbalanced force?

We will first consider a physics book at rest on a table top.

There are two forces acting upon the book.

One force - the Earth's gravitational pull - exerts a downward force.

The other force - the push of the table on the book pushes upward on the book.

Since these two forces are of equal magnitude and in opposite directions, they balance each other.

The book is said to be at equilibrium.

There is no unbalanced force acting upon the book and thus the book maintains its state of motion.

When all the forces acting upon an object balance each other, the object will be at equilibrium; it will not accelerate.

Now consider a book sliding from left to right across a table top.

The book is in motion and at the moment there is no one pushing it.

The forces acting upon the book are shown to the right.

Are the forces on the book balanced? Why or why not?

Unbalanced forces cause acceleration.

In this case, the unbalanced force is directed opposite the book's motion and will cause it to slow down.

Forces Are Unbalanced

Object accelerates