Tension in a cable car's cable

1. Jun 18, 2008

accobos

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 the figure. A 2000 kg block is attached to the tensioning pulley's cart, via a rope and pulley, and is suspended in a deep hole.

What is the tension in the cable car's cable?

T$$_{b on a}$$=T$$_{a on b}$$

I just don't even know where to start. I know that it's not accelerating, so Net Force=0 but I still can't figure out where to go from here. If I could just get pointed in the right direction, that'd be great! Thanks so very much!

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2. Jun 18, 2008

Staff: Mentor

Hint: Treat the cart and pulley as a single system. What forces act on it?

3. Jun 18, 2008

accobos

The tension of the hanging mass and the normal force of the rail on the cart, and the tension from the rope in the pulley?