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Tension and Pulleys

# Tension and Pulleys - experience two equal and opposite...

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Tension and Pulleys The dynamics of a single rope used to transmit force is clearly quite simple: the rope just transmits an applied force. When pulleys are used in addition to ropes, however, more complicated situations can arise. In a dynamical sense, pulleys simply act to change the direction of the rope; they do not change the magnitude of the forces on the rope. Just as we assumed the ropes to be massless, we will similarly assume that the pulleys we work with are massless and frictionless, unless told otherwise. The simplest case involving a pulley involves a block being lifted by another block connected to a rope: Figure %: The Tension in a Rope and Pulley System This diagram represents a small block on the left in the act of being lifted by a larger block on the right. Notice the forces T and -T: even when used in addition to a pulley, the rope must still
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Unformatted text preview: experience two equal and opposite tension forces. From the figure it may seem that the rope actually experiences two forces in the same direction, making the situation impossible. The presence of the pulley, however, changes the situation to make it physically tenable. When analyzing a rope and pulley situation it is useful to define a direction not in terms of up or down, but in terms of the shape of the rope. In the situation above, we can define the positive direction on the rope as pointing upward on the left side of the pulley, and pointing downward on the right side. When we define direction in this way the rope does actually experience two equal and opposite forces....
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