(a) N w E Pull rope in this direction S (b) The sum of the force vectors is zero. FIGURE P4.74
If your car is stuck in the mud and you don’t have a winch to pull it out, you can use a piece of rope and a tree to do the trick. First, you tie one end of the rope to your car and the other to a tree, then pull as hard as you can on the middle of the rope, as shown in P4.74a. This technique applies a force to the car much larger than the force that you can apply directly. To see why the car experiences such a large force, look at the forces acting on the center point of the rope, as shown in P4.74b. The sum of the forces is zero, thus the tension is much greater than the force you apply. It is this tension force that acts on the car and, with luck, pulls it free.
The sum of the three forces acting on the center point of the rope is assumed to be zero because
A. This point has a very small mass.
B. Tension forces in a rope always cancel.
C. This point is not accelerating.
D. The angle of deflection is very small.
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