Strings and ropes often pass over pulleys. The application might be as simple as lifting a heavy weight or as complex as the internal cable-and-pulley arrangement that precisely moves a robot arm.

FIGURE 7.24a shows a simple situation in which block B, as it falls, drags block A across a table. As the string moves, static friction between the string and pulley causes the pulley to turn. If we assume that

• The string and the pulley are both massless, and

• There is no friction where the pulley turns on its axle,

then no net force is needed to accelerate the string or turn the pulley. Thus the tension in a massless string remains constant as it passes over a massless, frictionless pulley.

FIGURE 7.24 Blocks A and B are connected by a string that passes over a pulley.

Because of this, we can draw the simplified free-body diagram of FIGURE 7.24b, in which the string and pulley are omitted. Forces $\vec{T}{}_{\text{A on B}}$ and $\vec{T}{}_{\text{B on A}}$ act as if they are an action/reaction pair, even though they are not opposite in direction because the tension force gets “turned” by the pulley.