Review AP Physics 1 Unit 2 to build your core toolkit for analyzing forces, free-body diagrams, Newton's three laws, friction, spring forces, and circular motion. This unit carries 18-23% of the exam and underpins nearly every dynamics problem you will encounter.
Use the topic guides, key terms, and practice questions available here to work through each concept before moving to Unit 3.
Unit 2 is the force and dynamics core of AP Physics 1. Every topic connects to a central question: what forces act on a system, and what do those forces cause the system to do? You will move from defining a system and locating its center of mass, to drawing free-body diagrams, to applying Newton's laws quantitatively across a wide range of scenarios.
Topics 2.1-2.2 establish the language of the unit. You choose a system, locate its center of mass using x_cm = (sum of m_i x_i)/(sum of m_i), and then draw every external force as a vector arrow originating from the center-of-mass dot. The coordinate system you choose, ideally with one axis along the direction of acceleration, determines how cleanly you can write the force equations.
Topics 2.3-2.5 give you the three laws in sequence. Newton's third law (F_A on B = -F_B on A) identifies paired forces on different objects. Newton's first law (sum F = 0 means constant velocity) defines translational equilibrium. Newton's second law (a_sys = F_net / m_sys) connects net external force to acceleration of the center of mass.
Topics 2.6-2.9 apply the laws to four specific force contexts: universal gravitation (F_g = Gm_1m_2/r^2), kinetic and static friction (F_f,k = mu_k N; F_f,s is less than or equal to mu_s N), spring restoring forces (F_s = -k delta x), and circular motion where the net inward force produces centripetal acceleration (a_c = v^2/r). Kepler's third law (T^2 = 4pi^2 R^3 / GM) extends circular motion to orbital satellites.
Every force in this unit is an interaction between two objects. Newton's third law guarantees a paired force on the other object. Newton's second law tells you what the net of all external forces does to the center of mass of your chosen system. Choosing your system carefully, drawing a complete free-body diagram, and summing forces along each axis are the three moves that unlock every problem in Unit 2.
Define a system, distinguish internal from external forces, and calculate the center of mass position using x_cm = (sum m_i x_i)/(sum m_i). Symmetric objects have their center of mass on the symmetry axis.
Represent every external force on an object as a vector arrow from a center-of-mass dot. Choose a coordinate system aligned with the acceleration direction to simplify force equations.
Every force has a paired force equal in magnitude and opposite in direction acting on the other object. Ideal strings have uniform tension; ideal pulleys redirect force without changing its magnitude.
When the net force on a system is zero, its velocity is constant (translational equilibrium). This applies to objects at rest and objects moving at constant velocity in an inertial reference frame.
A nonzero net external force causes the center of mass to accelerate: a_sys = F_net / m_sys. Apply this axis by axis using the free-body diagram to find unknown forces or accelerations.
Gravitational force between two masses follows F_g = G m_1 m_2 / r^2. Near Earth's surface, weight = mg. Apparent weight equals the normal force and changes when an object accelerates vertically.
Kinetic friction has fixed magnitude F_f,k = mu_k N and opposes sliding. Static friction adjusts up to F_f,s,max = mu_s N to prevent sliding. Both depend on the normal force, not contact area.
Hooke's law: F_s = -k delta x. The spring constant k (N/m) measures stiffness, and the restoring force always points toward the equilibrium position regardless of whether the spring is stretched or compressed.
Centripetal acceleration a_c = v^2/r points inward and is produced by real forces. Identify which forces contribute inward net force. For satellites, Kepler's third law T^2 = (4 pi^2/GM) R^3 connects period and orbital radius.
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Review Systems and Center of Mass with attention to how the concept appears in AP-style source and evidence questions.
Review Circular Motion with attention to how the concept appears in AP-style source and evidence questions.
Review Kinetic and Static Friction with attention to how the concept appears in AP-style source and evidence questions.
Review Newton's Third Law with attention to how the concept appears in AP-style source and evidence questions.
A system is any object or group of objects you choose to analyze together. Internal forces between parts of the system do not change the motion of the system's center of mass; only external forces do. The center of mass is the mass-weighted average position of the system, and you can treat the entire system as a single particle located there.
| Feature | Internal Force | External Force |
|---|---|---|
| Source | Between objects inside the system | From an object outside the system |
| Effect on center of mass | None; they cancel in Newton's third law pairs | Accelerates the center of mass |
| Appears on system FBD? | No | Yes |
A force is a vector interaction between two objects. Free-body diagrams (FBDs) show every external force on one chosen object as an arrow starting from a dot representing the center of mass. Drawing a correct FBD is the first step in applying any of Newton's laws.
| Force type | Symbol | Direction | When it acts |
|---|---|---|---|
| Weight | mg or F_g | Straight down toward Earth | Always, on any object with mass |
| Normal force | F_n | Perpendicular to and away from surface | When object contacts a surface |
| Tension | T | Along string, away from object | When object is connected to a string or rope |
| Friction | F_f | Parallel to surface, opposing relative motion or tendency | When surfaces are in contact |
| Spring force | F_s = -k delta x | Toward equilibrium position | When object is attached to a spring |
Newton's third law states that when object A exerts a force on object B, object B exerts an equal and opposite force on object A. These paired forces always act on different objects, so they never cancel each other on a single free-body diagram. Ideal strings transmit tension uniformly; ideal pulleys redirect force without changing its magnitude.
| Property | Ideal string | String with nonnegligible mass |
|---|---|---|
| Mass | Negligible (zero) | Has measurable mass |
| Tension along length | Same at all points | Varies along the string |
| Used in AP Physics 1 | Yes, standard assumption | Qualitative reasoning only |
Newton's first law: if the net force on a system is zero, its velocity stays constant (translational equilibrium). Newton's second law: if the net force is nonzero, the center of mass accelerates in the direction of that net force, with magnitude a = F_net / m_sys. These two laws cover every translational motion scenario in the unit.
| Condition | Net Force | Velocity | Acceleration |
|---|---|---|---|
| Static equilibrium | Zero | Zero (at rest) | Zero |
| Dynamic equilibrium | Zero | Constant, nonzero | Zero |
| Accelerating system | Nonzero | Changing | Nonzero, same direction as F_net |
Newton's law of universal gravitation gives the attractive force between any two masses: F_g = G m_1 m_2 / r^2. Near Earth's surface, the gravitational field is approximately uniform at g = 10 N/kg, so weight = mg. Apparent weight (the normal force you feel) differs from true weight whenever you accelerate vertically.
Friction is a contact force parallel to the surface. Kinetic friction acts when surfaces slide relative to each other and has a fixed magnitude. Static friction acts when surfaces are not sliding and adjusts up to a maximum value to prevent motion. Neither type depends on the area of contact.
| Property | Static Friction | Kinetic Friction |
|---|---|---|
| When it acts | Surfaces not sliding relative to each other | Surfaces sliding relative to each other |
| Magnitude | Adjusts; F_f,s is less than or equal to mu_s F_n | Fixed; F_f,k = mu_k F_n |
| Direction | Opposes tendency of motion | Opposes direction of relative motion |
| Coefficient | mu_s (typically larger) | mu_k (typically smaller) |
An ideal spring exerts a restoring force proportional to its displacement from its relaxed length: F_s = -k delta x. The negative sign means the force always points back toward the equilibrium position. The spring constant k (units: N/m) measures stiffness; a steeper slope on a force-vs-displacement graph means a stiffer spring.
An object moving in a circle has centripetal acceleration directed toward the center: a_c = v^2/r. This acceleration is produced by the net inward component of real forces (gravity, tension, normal force, friction). For vertical loops, the critical minimum speed at the top occurs when gravity alone provides the centripetal force. For satellites in circular orbit, Kepler's third law relates period and orbital radius to the mass of the central body.
| Scenario | Forces providing centripetal acceleration | Key equation |
|---|---|---|
| Horizontal circle on flat road | Static friction inward | mu_s m g = m v^2/r |
| Top of vertical loop | Gravity plus normal force (both inward) | mg + N = m v^2/r |
| Bottom of vertical loop | Normal force inward, gravity outward | N - mg = m v^2/r |
| Satellite in circular orbit | Gravity only | G M m / r^2 = m v^2/r |
| Banked curve (no friction) | Normal force component inward | N sin(theta) = m v^2/r |
Try stimulus-based AP practice questions and written prompts after you review the notes.
| Term | Definition |
|---|---|
| system-environment interaction | The exchange of energy or mass between a chosen system and its surroundings; external forces from the environment can change the motion of the system's center of mass, while internal forces cannot. |
| system equation | An equation describing the motion of an entire system by applying Newton's second law with the total system mass and the net external force; internal forces cancel and do not appear. |
| Newton's law of universal gravitation | F_g = G m_1 m_2 / r^2; the gravitational force between two masses is attractive, acts along the line connecting their centers of mass, and decreases with the square of the distance between them. |
| tangential speed | The instantaneous speed of an object moving along a circular path, directed tangent to the circle at that point; used with radius to calculate centripetal acceleration via a_c = v^2/r. |
| banked surface | An inclined circular path where the normal force component directed inward, and sometimes static friction, together supply the net centripetal force needed for circular motion. |
| Kepler's third law | T^2 = (4 pi^2 / GM) R^3; for a satellite in circular orbit, the square of the orbital period is proportional to the cube of the orbital radius, with M being the mass of the central body. |
| orbital period | The time T for a satellite to complete one full revolution in a circular orbit; related to orbital radius and central body mass by Kepler's third law. |
| orbital radius | The distance R from the center of the central body to the orbiting satellite; determines both the gravitational force on the satellite and its orbital period via Kepler's third law. |
Third-law paired forces act on different objects. They appear on different free-body diagrams and never cancel each other. Only forces on the same object can cancel when finding net force.
On an inclined plane, the normal force is mg cos(theta), not mg. Always resolve weight into components perpendicular and parallel to the surface before writing friction equations.
Centripetal force is not a new force you add to the free-body diagram. It is the label for the net inward force produced by real forces already on the diagram, such as gravity, tension, or friction.
Static friction adjusts to whatever value is needed to prevent sliding, up to mu_s times N. It is not always at its maximum value; only set F_f,s = mu_s N when the object is on the verge of slipping.
Delta x in F_s = -k delta x is measured from the spring's relaxed (natural) length, not from an arbitrary position. Measuring from the wrong reference gives the wrong force magnitude and direction.
AP Physics 1 frequently asks you to draw a free-body diagram for a specific object in a multi-object scenario, label each force with its type and agent, and then use the diagram to write Newton's second law equations. You may also be asked to explain why a particular force does or does not appear, or to compare diagrams for two objects interacting via Newton's third law.
A common task pattern presents a physical scenario and asks you to predict how a change in one variable (mass, applied force, surface angle, orbital radius) affects another quantity (acceleration, friction force, normal force, orbital period). You need to reason from the relevant equation and explain the direction of the change, not just calculate a number.
Exam questions often require you to move between a written description, a free-body diagram, and an algebraic equation within a single problem. Circular motion and inclined-plane problems in particular require you to identify which real forces contribute to the net centripetal or net parallel force, set up the correct Newton's second law equation, and interpret the result in terms of the physical situation.
Open the individual guides for Unit 2 when you want a closer review of one topic.
browse guidesPractice free-response reasoning and compare your answer with scoring guidance.
practice FRQsWatch past review streams filtered to Unit 2 when you want a video walkthrough.
open videosUse unit cheatsheets for a quick visual review after you work through the notes.
open cheatsheetsEstimate your broader AP score goal after you review the course and exam format.
open calculatorAP Physics 1 Unit 2 covers 9 topics across force and translational dynamics: Systems and Center of Mass, Forces and Free-Body Diagrams, Newton's Third Law, Newton's First Law, Newton's Second Law, Gravitational Force, Kinetic and Static Friction, Spring Forces, and Circular Motion. Free-body diagrams connect nearly every topic, so getting comfortable drawing them early pays off across the whole unit. See all 9 topics at AP Physics 1 Unit 2.
Unit 2 makes up 18-23% of the AP Physics 1 exam, making it one of the heaviest-weighted units on the test. It covers force and translational dynamics, including friction, gravitational force, Newton's three laws, spring forces, and circular motion. Expect multiple MCQs and at least one FRQ that draws from these concepts.
The AP Physics 1 Unit 2 progress check includes both MCQ and FRQ parts that test your understanding of force and translational dynamics. The MCQ section pulls from topics like Newton's laws, free-body diagrams, gravitational force, friction, and circular motion. The FRQ part typically asks you to draw and interpret free-body diagrams, apply Newton's Second Law, or analyze a scenario involving kinetic and static friction or spring forces. Working through the progress check is one of the best ways to spot gaps before the real exam. Find matched practice at AP Physics 1 Unit 2.
AP Physics 1 Unit 2 FRQs most often focus on free-body diagrams, Newton's Second Law (net force equals mass times acceleration), friction scenarios, and circular motion. Question types include Experimental Design, Qualitative/Quantitative Translation, and Short Answer. To practice, draw a complete free-body diagram for every problem before writing any equations, then show each step of your algebra clearly since College Board awards points for process, not just the final answer. Practice Unit 2 FRQs at AP Physics 1 Unit 2.
For AP Physics 1 Unit 2 practice questions, including multiple-choice and practice test sets, head to AP Physics 1 Unit 2. You'll find MCQs covering Newton's laws, friction, gravitational force, spring forces, and circular motion, plus FRQ practice with worked explanations. Mixing MCQ drills with full free-response attempts is the most effective way to prepare for the 18-23% of the exam this unit represents.
Start AP Physics 1 Unit 2 by building a strong foundation in free-body diagrams before anything else. Every topic from friction to circular motion depends on correctly identifying and drawing all forces on a system. From there, work through Newton's three laws in order, then apply them to gravitational force, kinetic and static friction, and spring forces before tackling circular motion, which combines several earlier ideas. Here's a practical study sequence: 1. Practice drawing free-body diagrams for simple systems (Topic 2.2) until it feels automatic. 2. Solve Newton's Second Law problems (Topic 2.5) with one force, then layer in friction (Topic 2.7) and gravitational force (Topic 2.6). 3. Work circular motion problems (Topic 2.9) by identifying the net centripetal force from your free-body diagram. 4. Do timed FRQ practice and check that every step of your algebra is visible on paper. All topics and practice sets are at AP Physics 1 Unit 2.