The law of inertia, sometimes referred to as Newton’s first law, states the following:

Source: Marble, wjlonien, Wikimedia Commons

Let’s look at an example. While looking at the example, think about how it relates to the law of inertia.

The Mind Bending Marble

In their Physics class, John tried to impress Tina by using the powers of his mind. He claimed that he could make a marble move across the counter by simply staring at it and concentrating on the word "move." Unfortunately, John was unsuccessful.

The next day in class, Tina told John to try something that should have been easier. She knew that the counter top was level, so she told John that she would roll the marble slowly across the counter. All that he had to do was concentrate on making the marble change speed or direction using the power of his mind. When he tried again, John noticed that the marble kept rolling in a straight line as long as it was on the counter top. Tina gave John another chance to impress her by asking him to explain what happened. She gave him the hint to look in his Physics book.

That evening, John looked at the chapter on forces and learned something he knew would impress Tina. The next day in class, he explained what happened with the marble. What do you think that John learned about Newton's first law from his physics book?

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Newton's First Law

An object at rest remains at rest, and an object in motion remains in motion at constant velocity, until acted upon by a net external force. When the marble was sitting on the table, there was not an unbalanced force acting on it, so it did not move. When Tina tapped on the marble, she introduced an unbalanced force causing the marble to move at a constant velocity.

Challenge question: What do you think should stop the marble?

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Check Your Answer

There are three possible ways the marble could be stopped: if John added an unbalanced force by putting his hand down, if the marble fell off the table, or if the marble ran into a book on the table.

Calculating the Forces Acting on an Object

Now, let’s take a brief look at how forces act on an object to better assist you with understanding the upcoming laws of motion.

When considering the forces acting on an object, you should consider all of the forces that act on that object. Look at the images below while considering the following:

What types of forces are acting on each object?
Is there only one force acting on each object?
Are these forces equal?

Interactive popup. Assistance may be required. There is more than one force acting upon the object including gravity, force of engines, and friction.

Interactive popup. Assistance may be required. There is more than one force acting upon the object including gravity, force the girl’s mass, and friction.

Interactive popup. Assistance may be required. There is more than one force acting upon the object including gravity and the upward force of the rock below.

To calculate the net force working upon an object, you must consider all of the forces acting upon that object and the magnitude of each. Watch the video below for an example of calculating the force on an object.

Now that you have had an opportunity to look at forces on an object, in the next section you will look at the relationship between force, mass, and acceleration.