The cable cars in San Francisco are pulled along their tracks by an underground steel cable that moves along at 9.5 mph . The cable is driven by large motors at a central power station and extends, via an intricate pulley arrangement, for several miles beneath the city streets. The length of a cable stretches by up to 100 ft during its lifetime. To keep the tension constant, the cable passes around a 1.5-m -diameter "tensioning pulley" that rolls back and forth on rails, as shown in (Figure 1). A m = 2000 kg block is attached to the tensioning pulley's cart, via a rope and pulley, and is suspended in a deep hole.
