AP Physics C 2017

F (^) net for a Pulley
Before we present our next practice problem, a few words about tension and pulleys are in order. Tension
in a rope is the same everywhere in the rope, even if the rope changes direction (such as when it goes
around a pulley) or if the tension acts in different directions on different objects. ONE ROPE = ONE
TENSION. If there are multiple ropes in a problem, each rope will have its own tension. TWO ROPES =
TWO TENSIONS.^3
When masses are attached to a pulley, the pulley can only rotate one of two ways. Call one way
positive, the other, negative.
A block of mass M and a block of mass m are connected by a thin string that passes over a light
frictionless pulley. Find the acceleration of the system.
We arbitrarily call counterclockwise rotation of the pulley “positive.”
Step 1 : Free-body diagrams.
The tension T is the same for each block—ONE ROPE = ONE TENSION. Also, note that because the
blocks are connected, they will have the same acceleration, which we call a .
Step 2 : Components.
The vectors already line up with one another. On to Step 3 .