Physics 1 · Unit 2: Dynamics · 14 min read · Updated 2026-05-11
Friction and Tension — AP Physics 1
AP Physics 1 · Unit 2: Dynamics · 14 min read
1. Core Definitions of Friction and Tension★☆☆☆☆⏱ 2 min
Friction is a contact force that opposes relative motion between two solid surfaces in contact, while tension is a pulling force transmitted through a flexible stretched medium (e.g., a rope, string, or cable). This subtopic makes up roughly a third of AP Physics 1 Unit 2: Dynamics, which accounts for 12–18% of your total AP exam score, appearing regularly in both multiple-choice and free-response sections.
For AP Physics 1, we almost always assume ideal ropes (massless, inextensible) and ideal pulleys (massless, frictionless) unless explicitly stated otherwise. This simplifies analysis because tension is uniform along an ideal rope.
2. Static and Kinetic Friction★★☆☆☆⏱ 4 min
Friction is split into two categories based on whether surfaces are moving relative to each other. Static friction acts when there is no relative motion, and adjusts its magnitude to exactly oppose the parallel component of the applied force, up to a maximum threshold. Kinetic friction acts when surfaces slide relative to each other, and has a constant magnitude for a given surface pair and normal force.
The formula for maximum static friction is:
f_{s,max} = \mu_s N
where $\mu_s$ is the dimensionless coefficient of static friction (dependent on the two surface materials), and $N$ is the magnitude of the normal force perpendicular to the contact surface. Kinetic friction follows the formula:
f_k = \mu_k N
For any pair of surfaces, $\mu_k < \mu_s$, which means it takes more force to start moving an object than to keep it moving at constant speed. A common misconception is that normal force always equals an object’s weight; this is only true for horizontal surfaces with no additional vertical forces. $N$ must always be calculated from Newton’s second law in the direction perpendicular to the contact surface.
3. Tension in Ideal Ropes and Pulleys★★☆☆☆⏱ 3 min
Tension is a pulling force that acts along the length of a rope, pulling equally on both objects connected to the rope. For AP Physics 1, all ropes and pulleys are assumed ideal unless stated otherwise, with the following properties:
Ideal rope: massless and inextensible. Inextensible means all connected objects have the same magnitude of acceleration, even if acceleration directions differ. Massless means net force on the rope is zero, so tension is uniform along the rope.
Ideal fixed pulley: massless and frictionless. It only changes the direction of tension, not its magnitude, so tension is equal on both sides of the pulley.
4. Combined Tension-Friction Connected Systems★★★☆☆⏱ 5 min
Most AP Physics 1 problems involving both friction and tension are connected object systems, where one or more objects rest on a frictional surface, connected by a rope and pulley to a hanging object. Follow this systematic approach to solve these problems:
Draw a separate free-body diagram for every object in the system
Resolve forces into components aligned with the direction of possible motion
Write Newton's second law for each object, using equal tension and equal acceleration magnitude for ideal systems
Check if the system is stationary or accelerating by comparing the applied pulling force to maximum static friction, then solve the system of equations
Common Pitfalls
Why: Students memorize the maximum static friction formula and apply it to all static friction cases, forgetting static friction adjusts to match the applied force
Why: Students generalize from simple horizontal surface problems to all cases, including angled forces and inclines
Why: Students confuse different acceleration directions with different magnitudes of acceleration
Why: Students assume pulleys change tension magnitude, when they only change direction for ideal fixed pulleys
Why: Students rush to use the kinetic friction formula without checking if motion actually occurs