These 2 Physics Videos Can Help You Understand Newton's 3rd Law of Motion

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By Andrew Bennett

You’ve likely heard of Sir Isaac Newton’s 3rd Law of Motion, "For every action, there is an equal and opposite reaction." But what does it really mean?

What Is Newton's 3rd Law of Motion?

Many people (even teachers) wrongly think that Newton’s 3rd Law is somehow linked to cause and effect and that the effect is somehow the opposite of the cause. This is simply wrong.

What the law really means is that forces come in pairs. In other words, when two objects interact with each other, each object applies a force to the other. The forces the objects apply are equal in size and opposite in direction.

3rd Law Videos

Newton's 3rd Law of Motion Definition and Examples

In the first video, you will learn about the general principle of Newton’s 3rd Law. You will also learn a helpful naming system for forces that will help you identify 3rd Law pairs. Finally, you will see several examples of how 3rd Law pairs work.

Newton's 3rd Law of Motion Demo

Another misconception people have about the 3rd Law is: "If Newton's 3rd predicts an equal and opposite force to whatever force I apply, then the object I'm applying a force to could never move because the forces balance."

The second video shows why that thinking is incorrect with data collected from two carts connected by a string. In the demonstration, you will see that each cart experiences the same force in the string. However, the lead cart experiences a larger force in the string pulling it forward than in the string that connects it to the other cart.

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