Chapter 4, Problem 77MSPP

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 Figure P4.68 a. 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 Figure P4.68 b. 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.

Figure P4.68

71. Suppose your efforts work, and the car begins to move forward out of the mud. As it does so, the force of the car on the rope is

A. Zero.

B. Less than the force of the rope on the car.

C. Equal to the force of the rope on the car.

D. Greater than the force of the rope on the car.