Whenever two bodies interact, the forces they exert on each other are equal in size, act in opposite directions, and are of the same type
Newton's Third Law: The foot pushes the ground backwards, and the ground pushes the foot forwards
A physics textbook is at rest on a dining room table.
Eugene draws a free body force diagram for the book and labels the forces acting on it. Eugene says the diagram is an example of Newton's third law of motion. William disagrees with Eugene and says the diagram is an example of Newton's first law of motion.By referring to the free-body force diagram, state and explain who is correct.
Step 1: State Newton's first law of motion
Step 2: State Newton's third law of motion
Step 3: Check if the diagram satisfies the two conditions for identifying Newton's third law
Step 4: Conclude which person is correct
Remember that pairs of equal and opposite forces in Newton's third law act on two different objects. It's a really common mistake to confuse Newton's third law with Newton's first law. That also means it's a really common exam question, which often uses a version of thee example below.
Applying this check will help you distinguish between them.
Newton's first law involves forces acting on a single object.
These differences are shown in Scenario 1 (Newton's first law) vs. Scenario 2 (Newton's third law)