Tension is the pulling force transmitted through a string, rope, cable, or chain, acting along the connector's length. The AP CED defines it as the macroscopic net result of forces that segments of the string exert on each other; an ideal string is massless and doesn't stretch, so tension is the same everywhere in it.
Tension is what you feel when you tug on a rope. Microscopically, every tiny segment of the string pulls on its neighbors, and tension is the macroscopic net result of all those segment-on-segment forces responding to an external pull. That's the exact framing in the CED's essential knowledge for LO 2.3.A, and it's why tension is really a Newton's third law story. Each piece of string pulls back on the piece pulling it with equal magnitude.
On the AP exam you'll almost always use the ideal string model. An ideal string has negligible mass and does not stretch, which means the tension has the same magnitude at every point along it (and on both sides of an ideal, frictionless pulley). One more rule that saves you from wrong free-body diagrams. Strings can only pull, never push. So in any FBD, the tension vector always points away from the object, along the string.
Tension lives at the heart of Unit 2 (Force and Translational Dynamics). It's introduced as a contact force in Topic 2.3, supports LO 2.3.A on Newton's third law force pairs, and feeds directly into LO 2.5.A and Topic 2.6, where you sum forces (including tension) to find acceleration with Newton's second law. But it doesn't stay in Unit 2. Tension is the centripetal force in pendulum and string-swinging problems (Topics 3.7 and 3.8), it stores and transfers energy in pulley-spring systems, and in Unit 10 the tension in a string sets the speed of waves traveling on it (Topic 10.3). If you can draw tension correctly in a free-body diagram, you've unlocked a huge fraction of the dynamics problems on this exam.
Keep studying AP Physics 1 Unit 2
Newton's Third Law and Free-Body Diagrams (Unit 2)
Tension is Newton's third law in disguise. The string pulls the block toward the string, and the block pulls the string right back with equal magnitude. When a rope connects two blocks, the rope transmits that pull so the tension on each block forms a matched pair. Mislabeling which object each tension acts on is one of the most common FBD errors graders see.
Uniform Circular Motion (Unit 3)
When you swing a ball on a string, tension is the force pointing toward the center, so it supplies the centripetal acceleration. At the bottom of a vertical circle, tension must exceed gravity to curve the path upward, which is why T is largest there. Topics 3.7 and 3.8 test exactly this kind of reasoning.
Atwood Machine (Unit 2)
The Atwood machine is the classic tension setup. Two blocks hang from a string over a pulley, and the same tension acts on both. The key insight is that tension is an internal force if you treat both blocks as one system, so it cancels out and you can find acceleration from the external forces alone. The 2019 FRQ on two connected blocks rewards exactly this systems thinking.
Waves on Strings (Unit 10)
In Topic 10.3, tension switches jobs. Instead of accelerating blocks, it determines how fast a wave travels down the string. Tighter string means faster waves, which changes the standing-wave frequencies. Same force, completely different unit.
Tension is everywhere on this exam, usually as part of a bigger dynamics problem rather than a standalone question. The 2019 long FRQ asked how the relative masses of two connected blocks affect their acceleration, which requires finding tension in the connecting string. The 2022 short FRQ combined a string-pulley system with a spring, mixing tension into an energy and forces argument. The 2023 short FRQ wrapped a string around a massive pulley, where the string's tension provides the torque that spins the pulley. The pattern is clear. You need to (1) draw tension correctly in a free-body diagram, pointing along the string and away from the object, (2) apply Newton's second law to each object or to the whole system, and (3) recognize when tension is an internal force that cancels for a well-chosen system. MCQs love the conceptual traps, like assuming tension equals weight when the object is accelerating.
Tension and compression are opposite internal responses. Tension is a pull transmitted through a flexible connector like a rope, and it always points away from the object along the string. Compression is a push transmitted through a rigid object like a rod or beam. The practical difference on the exam is that a string can only pull, while a rigid rod can push or pull. If your FBD shows a rope pushing on something, that diagram is wrong.
Tension is the macroscopic net result of forces that segments of a string exert on each other, which is the CED's Newton's third law framing in LO 2.3.A.
An ideal string is massless and doesn't stretch, so the tension is the same magnitude at every point in the string and on both sides of an ideal pulley.
Strings only pull. In a free-body diagram, the tension vector always points away from the object, along the direction of the string.
Tension does not automatically equal an object's weight. If the object is accelerating, like at the bottom of a swing or in an Atwood machine, you must solve Newton's second law to find T.
If you choose a system containing both connected objects, tension becomes an internal force and cancels out, so the system's acceleration depends only on external forces.
Tension reappears in Unit 10, where it sets the speed of waves on a string and therefore the standing-wave frequencies.
Tension is the pulling force transmitted through a string, rope, cable, or chain, acting along its length. The CED defines it as the net result of the forces that segments of the string exert on each other, and the ideal string model (massless, unstretchable) means tension is the same at both ends.
No. Tension equals weight only when the object is in equilibrium. If the object accelerates, like the blocks in the 2019 FRQ on connected blocks, tension is less than the heavier block's weight and greater than the lighter block's weight. Always solve F_net = ma instead of assuming T = mg.
Tension is a pull through a flexible connector like a rope; compression is a push through a rigid object like a rod. On the exam, the rule of thumb is that a string can only pull, so tension always points away from the object in a free-body diagram.
Yes, as long as the string is ideal (massless and unstretchable) and any pulley is massless and frictionless. If a pulley has rotational inertia, like in the 2023 short FRQ, the string's tension is what provides the torque that spins it, and you have to account for that in your equations.
Yes. When an object swings in a circle on a string, the tension points toward the center and supplies the centripetal acceleration. That's why tension is greatest at the bottom of a vertical circle, where it must both support the weight and curve the path upward.