Tru Physics

Newton’s Third Law

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Forces come in pairs which are of the same magnitude but opposite direction.

Newton’s third law of motion, also known as the law of action and reaction, is one of the fundamental principles of classical mechanics. One of the statements of the third law is that forces come in pairs. This means that if object A exerts a force on object B, then object B will exert an equal and opposite force on object A.

The third law can be expressed mathematically as:

\vec{F}_A^{(B)} = - \vec{F}_B^{(A)}

where \vec{F}_A^{(B)} is the force exerted by object A which acts on object B, and \vec{F}_B^{(A)} is the force exerted by object B which acts on object A. The negative sign indicates that the forces are in opposite directions.

Sir Isaac Newton’s own words used to describe his third law (translated from the original Latin) are:

“To every action, there is always opposed an equal reaction; or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.“

Newton's third law says that the force exerted by the ball on the catcher's glove is the equal in magnitude to the force exerted by the catcher's glove back on the ball.
The force exerted by the ball on the catcher’s glove is the equal in magnitude to the force exerted by the catcher’s glove back on the ball.

The third law applies to all types of forces, including gravitational, electric, and magnetic forces. For example, the gravitational force between the Earth and the Moon is equal and opposite, according to the third law. Similarly, the gravitational force that the Earth exerts on you, the reader, is perfectly equal in magnitude to the gravitational force that you exert on the Earth. Both bodies exert an equal and opposite pull on the other.

It is important to note that the forces always act on different objects, not on the same object. This is a common misunderstanding. The forces do not cancel each other out, as they act on different objects. If the third law were to state that these two equal and opposite forces acted on the same object, acceleration could never occur as no object could ever have a net force. This is NOT the case.

The third law has important applications in many fields, including physics, engineering, and biology. For example, it is used in designing rockets and other propulsion systems, as well as in understanding how organisms move and interact with their environment.

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Comments

One response to “Newton’s Third Law”

  1. Newton’s Laws of Motion – Tru Physics

    […] The Third Law is quoted least often but is important nonetheless. It states that forces always come in pairs. Two objects, A and B, will always exert the same force on one another. If A is your left hand and B is your right hand, this law states that, if you push you hands together, neither one will be able to push harder against the other (no matter how you push). The same force that you exert on one will be returned to the other in equal magnitude and opposite direction. This is not intuitive. […]

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