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AP Physics 1: Algebra-Based · Unit 2 of 8
Force and Translational Dynamics
18–23% of the exam4 lessons · 54 min28 terms
What this unit covers
The topics below follow the published Physics 1 course framework for Unit 2. This unit is worth 18–23% of the exam, so budget your time against that rather than against how long the unit takes to teach.
Newton’s lawsFrictionFree-body diagramsCircular motion & gravitation
Lessons in this unit
- Newton’s First & Second Laws13 min · 3 objectivesState Newton’s first law and recognize when an object is in equilibrium · Apply Newton’s second law, ΣF = ma, to find net force or acceleration · Distinguish mass (inertia) from weight (a force)
- Newton’s Third Law & Free-Body Diagrams14 min · 3 objectivesState Newton’s third law and identify true action–reaction pairs · Draw a free-body diagram showing every force on one object · Resolve weight into components on an inclined plane
- Friction13 min · 3 objectivesDistinguish static friction from kinetic friction · Compute the friction force from f = μN · Decide whether an applied force is enough to start an object sliding
- Circular Motion & Gravitation14 min · 3 objectivesCompute centripetal acceleration from speed and radius · Identify which real force supplies the centripetal force in a situation · Apply Newton’s law of universal gravitation
Formulas in Unit 2
Newton’s second law
ΣF = ma
ΣF is the vector sum of all forces (newtons, N). Solve component by component: ΣF_x = ma_x and ΣF_y = ma_y.
Weight
W = mg
g = 10 m/s² here (AP tables use 9.8). Weight is a force in newtons; it is never measured in kilograms.
Weight components on an incline (angle θ)
along incline: mg sinθ · perpendicular: mg cosθ
On a ramp, gravity splits into a part that pulls the object down the slope (mg sinθ) and a part pressing it into the surface (mg cosθ).
Friction force
f_k = μ_k N · f_s ≤ μ_s N
On flat ground with no vertical push, N = mg. Static friction is an inequality: it only reaches its maximum right at the verge of slipping.
Centripetal acceleration and force
a_c = v² / r · F_c = mv² / r
Both point toward the center of the circle. Doubling the speed quadruples both, because v is squared.
Newton’s law of universal gravitation
F = G·m₁·m₂ / r²
G = 6.67 × 10⁻¹¹ N·m²/kg². The force is an inverse-square law: triple the separation and the force drops to one-ninth.
Every term in Unit 2
All 28 terms we publish for Force and Translational Dynamics, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.
- Newton's second law
- ΣF = ma, applied separately along each axis. Acceleration is in the direction of the NET force, not of any single force.
- Weight vs mass
- Mass is the amount of matter and is frame-independent; weight is the gravitational force mg and changes with location.
- Newton's third law
- Forces come in equal and opposite pairs acting on DIFFERENT objects. Two third-law forces can never cancel, since they act on different bodies.
- Newton's first law
- An object's velocity is constant unless a net external force acts. Inertia is the resistance to that change, measured by mass.
- Free-body diagram
- Every force acting on one object, drawn from its center. Only real interactions appear — no "force of motion".
- Normal force
- Perpendicular contact force from a surface. It equals mg only on a level surface with no vertical acceleration and no other vertical force.
- Static vs kinetic friction
- Static friction adjusts up to μsN to prevent sliding; kinetic friction is μkN and constant once sliding. μs is generally larger than μk.
- Friction direction
- Opposes relative sliding between the surfaces, which is not always opposite the object's motion — friction propels a walking person forward.
- Tension
- The pull transmitted along a string. In an ideal massless string over a frictionless pulley, tension is the same throughout.
- Inclined plane components
- Weight resolves into mg sin θ along the incline and mg cos θ perpendicular to it, so the normal force is mg cos θ.
- Terminal velocity
- Reached when drag equals weight, so net force and acceleration become zero and velocity stops increasing.
- Uniform circular motion
- Speed is constant but velocity changes direction, so there is a centripetal acceleration a = v²/r directed toward the center.
- Centripetal force
- Not a new force — it is the name for whatever real force points to the center: tension, gravity, friction or normal force.
- Newton's law of gravitation
- F = Gm₁m₂/r², where r is measured center to center. Doubling the separation quarters the force.
- Orbital motion
- Gravity supplies the centripetal force, so Gm₁m₂/r² = mv²/r. Orbital speed depends on the central mass and radius, not the orbiting mass.
- Apparent weight
- The normal force a scale reads. It exceeds mg when accelerating upward and falls to zero in free fall.
- Identifying third-law pairs
- The pair to "Earth pulls ball down" is "ball pulls Earth up", not "ground pushes ball up". Swap the two objects and reverse the direction.
- Why the normal force is not always mg
- It equals mg only when the surface is level and there is no vertical acceleration or other vertical force. In a lift or on an incline it is not.
- Atwood machine
- Two masses over a pulley accelerate together at a = (m₁ − m₂)g/(m₁ + m₂), with the same magnitude of acceleration and one tension throughout.
- Connected objects
- Treat the system as one body to find acceleration, then isolate one body to find the internal force between them.
- Banked curve
- On a frictionless banked curve, the horizontal component of the normal force supplies the centripetal force, so the required angle depends on speed and radius.
- Vertical circular motion
- At the top, gravity and any tension both point to the center. The minimum speed to maintain contact is where the normal force just reaches zero.
- Free-body diagram errors
- The commonest are drawing a "force of motion" in the direction of travel, and including centripetal force as a separate arrow alongside the real force causing it.
- Static equilibrium
- ΣF = 0 in every direction and Στ = 0 about every axis. Both conditions are required; forces alone are not sufficient for an extended body.
- Gravitational field strength
- g = GM/r², the force per unit mass. It explains why g differs on other planets and why it falls with altitude.
- Inertial mass vs gravitational mass
- Inertial mass resists acceleration, gravitational mass determines weight. They are experimentally identical, which underlies general relativity.
- Force as the slope of momentum
- Net force is the rate of change of momentum, so a force-time graph's area gives the momentum change directly.
- Why heavier objects do not fall faster
- Doubling mass doubles the gravitational force but also doubles the inertia resisting it, so a = F/m = g regardless of mass.
What examiners penalize here
- On the AP exam, "equilibrium" does not mean "at rest." An object moving at constant velocity has zero acceleration, so its net force is also zero. Constant velocity and rest are the *same* dynamical situation.
- Draw the free-body diagram before writing a single equation. On free response, a correct FBD with properly labeled forces earns points even if your algebra later slips — and it prevents you from inventing forces that are not there.
- Watch the difference between "is it moving yet?" and "how fast does it slow down?" Use **static** friction (an inequality, up to μ_s N) to test whether motion starts; use **kinetic** friction (a fixed μ_k N) once it is already sliding.
- When a circular-motion problem asks for a force, first ask "which real force points toward the center?" Then set that force equal to mv²/r. Naming the actual force (tension, friction, gravity, normal) is what earns the reasoning point.
Practice Physics 1
Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.
Questions about this unit
How much of the AP Physics 1: Algebra-Based exam is Unit 2?
Unit 2, Force and Translational Dynamics, is worth 18–23% of the Physics 1 multiple-choice section according to the published course framework. Across all 8 units that makes it one of the heaviest units on the exam, and worth front-loading.
What topics are covered in Physics 1 Unit 2?
Force and Translational Dynamics covers Newton’s laws, Friction, Free-body diagrams and Circular motion & gravitation. We publish 28 terms with definitions for this unit, all of them on this page.
How should I study Physics 1 Unit 2?
Read the 4 lessons below first — about 55 minutes — then drill the 28 terms in cram mode until you can produce each definition from memory rather than just recognize it. Recognition is what makes a unit feel finished when it is not. Finish with practice questions and read the explanation for every one you get right by elimination as well as the ones you miss.
All 8 units of AP Physics 1: Algebra-Based
Unit names, topics and exam weights follow the published College Board course framework for AP Physics 1: Algebra-Based. AP® is a trademark registered by the College Board, which does not endorse this site.