Newton’s First Law
In an inertial reference frame, an object or system keeps a constant velocity—including remaining at rest—when the net external force on it is zero.
What do I need to know for AP Physics 1 Topic 2.4?
Topic 2.4 Newton’s First Law: In an inertial reference frame, an object or system keeps a constant velocity—including remaining at rest—when the net external force on it is zero.
- Recognize translational equilibrium, distinguish constant speed from constant velocity, connect balanced force diagrams to zero acceleration, and reason in inertial frames.
- Practice target: 42 original MCQs and 4 original FRQs for this exact topic.
- Course alignment: current AP Physics 1 framework for the 2026–27 school year and May 2027 exam.
What is Newton’s First Law?
In an inertial reference frame, an object or system keeps a constant velocity—including remaining at rest—when the net external force on it is zero.
Physics terms this topic teaches directly
Use these as working definitions for the lesson. Each definition is paired with a physical situation so the vocabulary is connected to observable motion or interactions rather than memorized in isolation.
Newton’s first law
In an inertial frame, an object with zero net external force maintains constant velocity, including the special case of remaining at rest.
Inertia
Inertia is the tendency of an object to resist changes in its velocity; mass quantifies translational inertia.
Inertial frame
An inertial frame is a frame in which Newton’s first law is valid without adding fictitious forces.
Translational equilibrium
Translational equilibrium means the net external force is zero, so acceleration is zero and velocity is constant; the object may be at rest or moving uniformly.
Understand these ideas here before continuing
You should not have to leave this lesson just to decode its vocabulary. Each concept below is defined here, connected to the physics of this topic, and illustrated with a real situation. A related lesson link is provided only for deeper study.
Velocity
Meaning here: A vector rate of change of position. Constant velocity means both speed and direction remain unchanged.
How this topic uses it: Newton’s first law says velocity remains constant when net external force is zero. “Constant” includes both zero velocity and nonzero straight-line velocity with unchanged speed and direction.
Net external force
Meaning here: The vector sum of all external forces acting on the chosen system.
How this topic uses it: The law is controlled by the vector sum of external forces, not by whether individual forces are present. Several nonzero forces can cancel to produce zero net force.
Inertial frame
Meaning here: A reference frame in which an object with zero net force has constant velocity.
How this topic uses it: The familiar statement of Newton’s first law assumes an inertial frame. If the observer’s frame accelerates, apparent motion can require additional frame-dependent reasoning.
Equilibrium
Meaning here: A state with zero net force. Translational equilibrium allows either rest or constant nonzero velocity.
How this topic uses it: Translational equilibrium means zero net force and therefore zero acceleration. It does not require the object to be at rest; constant nonzero velocity is also equilibrium.
How Newton’s First Law works
Build the physical model first. The examples below connect the idea to situations you could actually observe before the page turns to equations.
Zero net force means zero acceleration, not zero motion
Newton's first law states that an object in an inertial reference frame keeps a constant velocity when the net external force is zero. “Constant velocity” includes rest as the special case v = 0. A hockey puck gliding across nearly frictionless ice can continue moving at constant speed in a straight line even though no forward force is required. The law therefore rejects the common idea that motion itself needs a continuing net force.
Equilibrium can be static or dynamic
When all external force vectors add to zero, the object is in translational equilibrium. A hanging sign at rest is static equilibrium; a car moving straight at constant velocity can be dynamic equilibrium if the driving and resistive forces balance. In both cases acceleration is zero. The important test is the vector sum of forces, not whether the object is moving. A free-body diagram should show how the forces balance in each direction.
The statement is tied to inertial reference frames
An inertial frame is one in which an object with zero net force has no acceleration. A smoothly cruising train is approximately inertial for many classroom problems; a rapidly accelerating or turning vehicle is not. Inside an accelerating bus, a passenger may feel as if an extra backward force appears, but in an inertial ground frame the explanation is that the passenger's body tends to maintain its previous velocity while the bus accelerates forward.
Core ideas with real physical examples
Each category keeps the physics statement and the real situation together, so the page does not reduce the topic to abstract definitions.
| Idea/type | What it means | Real example 1 | Real example 2 |
|---|---|---|---|
| Translational equilibrium | Net external force is zero, so acceleration is zero. | A hanging sign is motionless with support forces balancing weight. | A car cruising straight at constant speed has driving and resistive forces balanced. |
| Constant velocity | Velocity stays unchanged in both magnitude and direction. | A puck coasts straight across nearly frictionless ice. | A spacecraft far from significant external forces continues at constant velocity after engines shut off. |
| Rest as constant velocity | Zero velocity is one valid constant velocity. | A book resting on a table has zero acceleration because vertical forces balance. | A parked car on level ground remains at rest while support and gravitational forces balance. |
| Inertial frame | A frame in which Newton’s laws apply without adding fictitious forces. | A classroom at rest relative to Earth is usually approximated as inertial for basic lab motion. | An accelerating elevator is not strictly inertial during the acceleration interval. |
| Balanced on one axis, unbalanced on another | Force balance is evaluated independently by axis; zero net force on one axis does not require zero net force on every axis. | A projectile has no horizontal force in the no-drag model but still has a downward gravitational force and vertical acceleration. | A crate can have N−mg=0 vertically while a net horizontal pull accelerates it sideways. |
College Board focus for Topic 2.4
2.4.A · Describe motion in an inertial frame when net external force is zero.
Important boundary: Zero net force means constant velocity, which may be zero or nonzero.
Equations, variables, SI units and when they apply
| Relationship | Variables / SI units | When to use it | Real example |
|---|---|---|---|
| ΣF⃗ = 0 ⇒ a⃗ = 0 ⇒ v⃗ = constant | F: N; a: m/s²; v: m/s | Use for equilibrium/constant-velocity situations. | A 20 N pull right balanced by 20 N friction left gives zero horizontal acceleration. |
| ΣFx=0, ΣFy=0 | Force components: N | Apply equilibrium separately along chosen perpendicular axes. | A static sign supported by angled cables must have horizontal and vertical force components summing to zero. |
Interactive Newton’s First Law check
Equilibrium missing-force solver
Enter three signed one-axis forces. The tool finds the fourth force needed for zero net force.
Adjust a slider to explore the model.
Three real-situation worked examples
Cruise control
A car travels east at steady speed on a level road.
Book at rest
A book remains on a table.
Constant-speed circle
A car circles a track at constant speed.
Common AP mistakes—and the correction
| Common mistake | Correct physics | Real-life check |
|---|---|---|
| “If an object is moving, a forward net force is required.” | A net force is required to change velocity, not to maintain constant velocity. | Cruise control: test the statement against this physical situation. |
| “Zero net force means zero velocity.” | It means zero acceleration; velocity can be any constant value. | Book at rest: test the statement against this physical situation. |
| “Constant speed always means equilibrium.” | Direction can change at constant speed, as in circular motion. | Constant-speed circle: test the statement against this physical situation. |
| “Balanced forces mean no forces are present.” | Several nonzero forces can cancel vectorially. | Cruise control: test the statement against this physical situation. |
Master this topic beyond a formula list
This checkpoint combines the current AP Physics 1 content boundary with representation, experiment/data reasoning, real situations and cross-topic transfer.
2026–27 mastery checklist
- Define an inertial reference frame for applying Newton’s first and second laws in their standard AP Physics 1 form.
- Infer constant velocity when net external force is zero.
- Recognize rest as the special case of constant velocity equal to zero.
- Distinguish balanced forces from absence of forces.
Must-not-miss relationship
ΣF_ext = 0 implies constant velocity in an inertial frame; it does not imply the object must be at rest.
Two anchor situations
A puck gliding on nearly frictionless ice can keep moving at constant velocity with almost zero net horizontal force.
A hanging sign remains at rest because upward support forces balance weight, not because no forces act.
Diagram / graph / equation / claim
Given several motion diagrams, identify which are compatible with zero net force and construct an FBD for each.
How to test the physics
Use a low-friction cart to compare motion before and after a brief push; analyze the near-constant-velocity interval after the interaction ends.
Apparent accelerations in a noninertial frame require special treatment outside this simple model; identify the frame before invoking equilibrium.
Use translational equilibrium here as the foundation for rotational equilibrium in Topic 5.5.
Connect to Topic 5.5 rotational equilibriumGo beyond memorizing the equation
This topic now combines model selection, physical intuition, a worked transfer problem, graph/data analysis, answer verification, and the four current AP free-response reasoning modes.
Choose the model before calculating
Check whether net external force is zero in an inertial frame. If it is, preserve the current velocity vector; do not automatically set velocity to zero.
Why the physics works
Zero net force means zero acceleration, so velocity does not change. Rest is only one possible constant velocity.
Solve this before opening the solution
A puck moves east at 6 m/s on nearly frictionless ice. If the net horizontal force is zero for 5 s, what is its velocity after 5 s?
Reveal complete reasoning
Solution: It remains 6 m/s east because acceleration is zero.
Physics check: A nonzero constant velocity is fully consistent with zero net force.
What should you plot?
Plot velocity versus time for an object under zero net force.
What does the graph mean?
The graph is horizontal. Its value may be zero, positive, or negative depending on the initial velocity.
How this exact topic can appear in each current FRQ mode
ΣF_ext = 0 implies constant velocity in an inertial frame; it does not imply the object must be at rest.
Given several motion diagrams, identify which are compatible with zero net force and construct an FBD for each.
Use a low-friction cart to compare motion before and after a brief push; analyze the near-constant-velocity interval after the interaction ends.
Use translational equilibrium here as the foundation for rotational equilibrium in Topic 5.5.
Use the chain ΣF=0 → a=0 → velocity constant, rather than the incorrect shortcut “no force means no motion.”
Continue with the exact topic question banks
Use the focused banks for skill practice. The five full model papers are reserved for whole-exam simulation.
Topic 2.4 quick questions
What is the first thing I should identify in a Topic 2.4 problem?
Recognize translational equilibrium, distinguish constant speed from constant velocity, connect balanced force diagrams to zero acceleration, and reason in inertial frames.
Should I memorize an equation before understanding the physical model?
No. Start by identifying the system, interactions, directions and representation. Then choose the relationship whose assumptions match the situation.
Where should I practice this exact topic?
Use the direct 42-MCQ and 4-FRQ Topic 2.4 banks on this page. The five-model-paper page is reserved for full-exam simulation.
Is this aligned to the May 2027 AP Physics 1 exam?
Yes. The course content remains based on the Fall 2024 framework for 2026–27; the Fall 2026 update changes the May 2027 exam timing and question count, not this Unit 2 topic sequence.
Where Topic 2.4 appears in AP-style released work
These are original topic summaries and alignment notes, not reproduced College Board question text. One released FRQ can legitimately connect to several topics.
Meterstick balance and torque experiment
Applies torque, equilibrium, measurement design and uncertainty.
Experimental Design & AnalysisHinged beam, support force and rotational motion
Combines torque, equilibrium and rotational kinematics.
Qualitative / Quantitative TranslationRecent public FRQs are the safest format reference. Older released questions can still contain excellent physics, but the course and exam changed; use the atlas alignment notes before treating a legacy question as a current-format simulation.