Why Gravity and Normal Force Aren't Action/Reaction Pairs

The concept of action/reaction pairs, rooted in Newton's Third Law of Motion, can sometimes lead to confusion when applied to physical scenarios involving gravity and the normal force. This article explores why these two forces do not form an action/reaction pair despite their importance in everyday physics.

Definition of Action/Reaction Pairs

According to Newton's Third Law of Motion, for every action there is an equal and opposite reaction. This means that the forces must act on two different objects. This definition is crucial to understanding why gravity and the normal force do not form an action/reaction pair.

Understanding Gravity and Normal Force

Gravity (Weight): The gravitational force, often referred to as weight, acts on an object due to the mass of the Earth or another massive body. For example, when a book is resting on a table, the gravitational force (weight) of the book pulls it downward toward the center of the Earth.

Normal Force: The normal force is the force exerted by a surface to support the weight of an object resting on it. In our example, the table exerts an upward normal force on the book, counteracting the book's weight.

Different Objects

In the book-table example:

The gravitational force (weight) acts on the book downward. The normal force acts on the table upward on the book.

Here, the forces do not act on the same object, which is a key requirement for action/reaction pairs. Therefore, gravity and the normal force are not considered an action/reaction pair.

Examples of Action/Reaction Pairs

Consider pushing against a wall. Your hand exerts a force on the wall (action), and in return, the wall exerts an equal and opposite force back on your hand (reaction). Both forces act on the same system (your hand and the wall).

Why the Gravitational and Normal Forces Don't Form an Action/Reaction Pair

While gravity and the normal force are related and can balance each other out in static situations like a book resting on a table, they do not form an action/reaction pair due to the distinct objects they act upon.

Further Explanation with Diagram

The diagram provided depicts two distinct action/reaction pairs of forces. The first pair is the gravitational forces acting on the body (G) and on the Earth (Grsquo;). The second pair is made by two pushing forces: the body pushes the Earth down with a force (F), and the Earth pushes the body up with a force (N), so N -F. The equality between G and F, N exists only because the displacement of the air produced by the body is negligible. However, as we submerge the body in water, the body will be pushed up by the buoyancy force (FG). In this case, F and N are not always equal to G.

This variability reinforces the idea that gravity and the normal force, while interconnected, do not form a clear action/reaction pair in all scenarios due to the different objects they act upon.

Conclusion

Gravity and the normal force are indeed related and can balance each other out in static situations, but they do not form an action/reaction pair because they act on different objects. Understanding this distinction is crucial for a clear grasp of fundamental physical principles.

By breaking down the mechanics of these forces and the requirements for action/reaction pairs, we can better comprehend the nuances of physical interactions. Whether you're studying the behavior of objects on Earth or considering more complex scenarios involving buoyancy and water displacements, the key is recognizing when forces act on the same object versus different ones.