One last topic to consider is that of pulleys. Pulleys are devices that change the direction of the tension force in cords connecting objects. Consider the two objects connected by a massless cord over the pulley. The forces on block #1 are gravity pointing down, the normal force pointing up, and the tension in the cord pointing away from the block, to the right. The forces on block #2 are the force of gravity, pointing down, and the tension force pointing up. Because the cord connecting the two blocks is massless, and because we assume the pulley to be massless and frictionless, the two tension forces are equal in magnitude. The change in direction of tension occurs because of the pulley. Let's write the components of the net force in both blocks. For block #1, we examine the x-component of the force, which is given by Fx and is equal to the tension force, which is equal to the mass of block #1 times the acceleration of the block. For block #2, we have forces in the y-direction only. These are gravity and the tension force. We therefore write the sum of the forces in the y-direction as the tension force minus the force of gravity, and this equals the mass of block #2 times the acceleration of the block. ... Since the two blocks are connected by a finite length of cord, the magnitude of acceleration is the same for both blocks. We can now solve these equations by substituting for the tension force in the second equation and solving for the acceleration. So, the acceleration is a fraction of what is would be if block #2 was falling due to gravity.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education