Newton's third law describes every interaction between two objects or systems in terms of a matched pair of forces: if object A exerts a force on object B, then object B exerts an equal-magnitude, opposite-direction force back on A. Neither force causes the other — they exist simultaneously, as two faces of one interaction.
This is exactly why normal force and gravity, both acting on a block resting on a table, are not a third-law pair — even in the common case where they happen to be equal in magnitude.
Try it yourself below — three classic scenarios, each with its two paired forces drawn on the two different objects involved.
Pick a scenario — both force arrows are always equal in length and opposite in direction, each one acting on a different object.
In a tug-of-war, the tension in the rope is genuinely the same at both ends (it's one ideal string). So why does one side usually still win?
Because every internal interaction inside a system comes as a third-law pair, those forces always cancel out when you sum up the net force on the whole system. Internal forces cannot change a system's center-of-mass motion — only external forces can.
Tension isn't a fundamental force of its own — it's the macroscopic, net result of countless infinitesimal segments of a string, cable, or chain pulling on their neighbors in response to an external force stretching the string taut.
An ideal string makes two simplifying assumptions that make tension problems tractable:
Real ropes have mass, and once they do, the "same tension everywhere" shortcut breaks down. A point higher up a hanging rope has to support not just whatever's hanging below it, but also the weight of all the rope between it and the bottom — so tension increases as you move up a massive hanging rope.
Compare the two cases directly below.
A rope holds a 5 kg mass, hung from a fixed support. Toggle between an ideal string (negligible mass) and a real rope with 3 kg of its own mass distributed along its length.
Ideal string: tension is identical at every point, top to bottom.
An ideal pulley makes its own pair of simplifying assumptions: negligible mass, and an axle that rotates with negligible friction. Together, these mean an ideal pulley doesn't change the magnitude of tension in a string passing over it — it only redirects the string's direction. This is exactly what lets you treat tension as a single shared value across every ideal string segment in a pulley system like the three-mass setup from this unit's header image.