A pulley is a grooved wheel that redirects a string or cable, changing the direction of the tension force without changing its magnitude, as long as the pulley is ideal (massless and frictionless). On AP Physics 1, pulleys link Newton's laws, tension, and system analysis in Unit 2.
A pulley is a wheel with a groove that a rope, string, or cable runs over. Its whole job is to change the direction of a force. Pull down on one side, and the string pulls up on whatever is attached to the other side. The force doing the pulling is tension, which the CED defines as the macroscopic net result of forces that segments of a string exert on each other in response to an external force.
In AP Physics 1, you'll almost always work with an ideal pulley: massless and frictionless. Combined with an ideal string (negligible mass, doesn't stretch), this gives you the two rules that make pulley problems solvable. First, the tension has the same magnitude everywhere in the string, on both sides of the pulley. Second, because the string doesn't stretch, objects connected by it share the same magnitude of acceleration. A pulley doesn't add force or energy. It just bends the path of the tension so a hanging weight can drag a block sideways across a table.
Pulleys live in Unit 2: Force and Translational Dynamics, mainly Topics 2.3 (Contact Forces) and 2.1 (Systems). They directly support learning objective 2.3.A, where you describe interactions between objects using Newton's third law and paired forces. Every segment of the string pulls on the next segment with equal and opposite forces, and that's exactly what tension is.
Pulleys are also the classic testing ground for 2.1.A and 2.1.B thinking about systems. Two blocks connected over a pulley can be analyzed two ways. You can draw a free-body diagram for each block separately (tension shows up as an internal force pair), or you can treat both blocks as one system, where the internal tension forces cancel and only external forces like gravity determine the acceleration of the system's center of mass. Being able to switch between those two views is one of the most valuable skills in Unit 2, and pulleys are how the exam checks it.
Keep studying AP Physics 1 Unit 2
Tension (Unit 2)
A pulley problem is really a tension problem in disguise. The pulley just redirects the string, so the same tension that pulls one block horizontally also holds up the hanging block. If the pulley and string are ideal, that tension magnitude is identical on both sides.
Systems and Center of Mass (Unit 2)
Two blocks over a pulley can be modeled as a single system. The tension forces are internal, and internal forces don't influence the motion of the system's center of mass. That lets you find the acceleration with one equation instead of solving two free-body diagrams simultaneously.
Friction and Normal Force (Unit 2)
Exam pulley setups love putting one block on a table or incline. Then the tension from the hanging block has to fight kinetic or static friction, which depends on the normal force. The 2022 short FRQ wired a pulley system to a spring on a frictional surface for exactly this reason.
Torque and Rotational Inertia (Unit 5)
Drop the "ideal" assumption and the pulley becomes a rotating disk with rotational inertia. Now the tensions on the two sides must differ, because a net torque is needed to angularly accelerate the pulley. The 2021 and 2023 FRQs both used massive pulleys to bridge translational and rotational dynamics.
Pulleys show up constantly on released FRQs. The 2019 long FRQ used a string over a pulley to explore how the relative masses of two blocks affect acceleration (a modified Atwood machine). The 2022 short FRQ connected a block on a surface to a hanging block over a pulley, with a spring added. The 2021 and 2023 short FRQs went further, using pulleys with mass and rotational inertia, where you have to apply both Newton's second law and the rotational version with torque.
What you actually have to do: draw accurate free-body diagrams for each object (tension points along the string, away from the object), write Newton's second law for each object or for the whole system, and recognize that connected objects share the same magnitude of acceleration. In multiple choice, watch for conceptual traps like "the tension equals the hanging block's weight" (only true if acceleration is zero). Always check whether the problem says the pulley is ideal. If the pulley has mass, the two tensions are different and you'll need torque.
An ideal pulley is massless and frictionless, so the tension is the same on both sides and the pulley is just a direction-changer. A pulley with mass has rotational inertia, so it needs a net torque to spin faster. That means the tension on one side must be larger than on the other. Misapplying the "equal tension" rule to a massive pulley is one of the most common errors on rotation FRQs like 2021 Q5 and 2023 Q4. Read the problem statement carefully before assuming the tensions match.
An ideal pulley changes the direction of the tension in a string without changing its magnitude.
An ideal string is massless and doesn't stretch, so objects connected by it over a pulley have the same magnitude of acceleration.
Tension is the net result of string segments pulling on each other, and those segment-to-segment pulls are Newton's third law force pairs (2.3.A).
You can solve a two-block pulley problem with separate free-body diagrams or by treating both blocks as one system, where tension is an internal force that cancels out.
The tension in the string only equals the hanging object's weight when the system isn't accelerating.
If the pulley has mass (rotational inertia), the tensions on the two sides are different, and you need torque from Unit 5 to solve the problem.
A pulley is a grooved wheel that redirects a string or cable. In Unit 2 problems, an ideal pulley (massless, frictionless) changes the direction of the tension force without changing its magnitude, letting a hanging weight pull a block horizontally.
Yes, but only if the pulley is ideal (massless and frictionless) and the string is ideal. If the pulley has mass, the tensions must differ to provide the net torque that angularly accelerates the pulley, which is exactly what the 2021 and 2023 short FRQs tested.
No, not when the system is accelerating. If the hanging block accelerates downward, the tension is less than its weight (T = mg minus ma). They're only equal when acceleration is zero. Assuming T = mg is one of the most common point-losers on these FRQs.
A single fixed pulley in AP Physics 1 gives no mechanical advantage; it only redirects the force, so input and output tension are equal. Mechanical advantage comes from multi-pulley systems that multiply force, which isn't the focus of AP Physics 1 pulley problems.
Either write Newton's second law for each block separately and solve the two equations together, or treat both blocks as one system so tension cancels as an internal force. Both blocks share the same magnitude of acceleration because the ideal string doesn't stretch. The 2019 long FRQ Q2 is a released example of this exact setup.
Connect this key term to the AP exam workflow: review the course, practice questions, and check related study tools.
Review units, study guides, and course resources.
Check this vocabulary in multiple-choice context.
Apply key concepts in written AP responses.
Estimate the exam score you are working toward.
Review the highest-yield facts before practice.
Put the full course together before test day.