A.2.8—Tension

Tension is a pull

Tension is the force exerted by a taut string, cable or rope on an attached body. It acts along the string and pulls away from the body.

Use the ideal-string model carefully

For a light, inextensible string over a frictionless pulley, tension has the same magnitude throughout. If the string, pulley or contact is non-ideal, tension can vary and must be found from each body’s force balance.

Connect tension to motion

Draw tension in the string direction, then use $\sum F=ma$. A body can have non-zero tension while at rest if other forces balance it.

Common trap

A string can pull but not push. Do not draw tension toward the string’s far end through the body or assume its value equals the weight without a force balance.