SAT Physics Subject Test

Chapter 3 Newton’s Laws

In the previous chapter we studied the vocabulary and equations that describe motion. Now we will learn why things move the way they do; this is the subject of dynamics.

An interaction between two bodies, a push or a pull, is called a force. You see examples of forces every day. If you lift a book, you exert an upward force (created by your muscles) on it. If you pull on a rope that’s attached to a crate, you create a tension in the rope that pulls the crate. When a skydiver is falling through the air, the earth is exerting a downward pull called gravitational force, and the air exerts an upward force called air resistance. When you stand on the floor, the floor provides an upward, supporting force called the normal force. If you slide a book across a table, the table exerts a frictional force against the book, so the book slows down and then stops. Static cling provides a directly observable example of the electrostatic force.

Sir Isaac Newton published a book in 1687 called Philosophiae Naturalis Principia Mathematica (Latin for “Mathematical Principles of Natural Philosophy”)—referred to nowadays as simply The Principia—that began the modern study of physics as a scientific discipline. To score well on force questions on the SAT Physics Subject Test, you will need to know about three of the laws that Newton stated in The Principia. These laws form the basis of dynamics and are known as Newton’s laws of motion.


Newton’s first law says that an object will continue in its state of motion unless compelled to change by a force impressed upon it.

If the object is at rest, then it will stay at rest, and if it is moving, then it will continue to move at a constant speed in a straight line.

The Skinny on the First Law

Mass is a measure of inertia; the more mass an object has the more the object resists changing its velocity. For example, if you hit a bowling ball with a bat, there’s not much change in the ball’s velocity. But if you hit a baseball with a bat with the same force, there’s a bigger change in velocity. Since a bowling ball has more mass, it has more inertia.

Basically, no force means no change in velocity. This property of objects—their natural resistance to changes in their state of motion—is called inertia. In fact, the first law is often referred to as the law of inertia.