T/F book at rest on TABLE. Statement: The 3rd law pair to force that Earth exerts on book is the force that the table exerts on book since it is equal and opposite reaction to force that Earth exerts.

Question

Is this statement correct or not and please explain why.

Answer 1

The statement is false. This is a common trick question on physics tests.

Newton's Third Law states that if object A exerts a force on object B, object B will exert an equal and opposite force on object A. The key here is the forces in a Newton's Third Law pair aren't only equal in magnitude/opposite in direction, but are also between the same two objects.

The trick to finding Newton's third law pair to a force is to reverse the "on/by" in the force. For example:

"The force exerted on the book by the Earth" is paired with "the force exerted by the book on the Earth".

So the pair force to the Earth pulling on the book is actually the gravitational force of the book pulling right back on the Earth. Of course, this force is a lot less noticeable because the mass of the Earth is enormous, so its acceleration from this specific force is both small and likely canceled out by other forces from books on the opposite side of the globe.

To check your understanding - what force can we put in a Newton's 3rd Law pair with the normal force exerted by the table on the book?

Let's apply the on/by reversal trick again, and see that the pair force is exerted by the book on the table. The surface of the book has its own Normal force, which is equal in magnitude and opposite in direction to the force from the table. If the table is placed on a floor, the normal force exerted by the floor on the table is paired with a normal force exerted by the table on the floor. Turtles all the way down.

This downward normal force might seem counterintuitive, but a common household object actually relies on this force to function - the bathroom scale! When you step on the scale, the scale exerts an upward normal force on your body, and your body exerts a downward normal force on the scale. The scale measures this force and reports it back to you as your weight.

A final trick to help you tell if two forces are in a Newton's 3rd Law pair - often the forces are the same type of force. I already talked about normal force and gravitational force, but this holds up for other force pairs as well. For example, if f I use tension to pull a rope, the rope is pulling back on me with tension.

Hope this helps, and let me know if you have any more questions!