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Browse the glossary using this index

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A

Action-reaction pairs

The force exerted (action) and the force experienced in return (reaction). Which is which depends on the perspective.

E

External forces

Forces acting on a system from outside; these determine the system's net force and acceleration.

F

Free-body diagram

A diagram showing a system of interest and all external forces acting on it.

I

Internal forces

Forces within a defined system; these do not affect the system's overall motion.

N

Newton's Third Law of Motion

Whenever a first object exerts a force on a second object, the first object experiences a force equal in magnitude but opposite in direction to the force that it exerts. Forces always occur in pairs.

Normal force

The force a surface exerts on an object, perpendicular to the surface, to support the object's weight. On a horizontal surface, it is equal in magnitude to the object's weight but opposite in direction. (N = mg)

S

System (or System of Interest)

The object or group of objects being analyzed.

T

Tension

The force along the length of a flexible connector (rope, string, etc.), acting parallel to the connector's length and pulling in opposite directions at both ends.

Thrust

The reaction force that propels a rocket or jet forward, resulting from expelling gas backward.