Newtons Third Law Calculator

Newton's Third Law of Motion

For every action force that object A exerts on object B, object B exerts an equal and opposite reaction force on object A. These forces act on different bodies, so they do not cancel when analyzing a single object.

Force Pair Rule

$$\vec{F}_{AB} = -\vec{F}_{BA}$$

Magnitudes are equal: $|\vec{F}_{AB}| = |\vec{F}_{BA}|$. Directions are opposite. Use this calculator to compute the reaction force from a known action force or to verify whether two entered forces form a valid action-reaction pair.

Examples

When you push a wall with 50 N eastward, the wall pushes you with 50 N westward. A book resting on a table pushes down; the table pushes up with equal magnitude.

Frequently Asked Questions

Do action and reaction forces cancel?

They cancel only when analyzing the combined system of both objects. On a single object, only the force acting on that object matters for its acceleration.

Must reaction forces be contact forces?

No. Third law pairs include gravitational, magnetic, and electrostatic forces, not only contact forces.

Why doesn't a book fall through a table?

The table exerts an upward normal force equal to the book's weight, so the net force on the book is zero and it stays at rest.

What makes a valid force pair?

Equal magnitudes, opposite directions, and the forces must act on two different objects in the same interaction.

How is this different from equilibrium?

Equilibrium means net force on one object is zero. Third law describes the paired forces between two interacting objects.

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Newtons Third Law Calculator

Newton's Third Law of Motion

Force Pair Rule

Examples

Frequently Asked Questions

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