Physics 1 · Unit 2: Dynamics · 14 min read · Updated 2026-05-11
Newton's First Law — AP Physics 1
AP Physics 1 · Unit 2: Dynamics · 14 min read
1. Newton's First Law and Inertia★★☆☆☆⏱ 4 min
Newton’s First Law of Motion (also called the Law of Inertia) is a core principle of AP Physics 1 Unit 2 Dynamics, which makes up 12-18% of total exam score. The formal definition is: *A body at rest remains at rest, and a body in motion with constant velocity (constant speed in a straight line) remains in motion with that constant velocity, if and only if the net external force acting on the body is zero*.
It is critical to distinguish between mass (inertia) and weight: mass is an invariant property of an object measured in kilograms, while weight is the gravitational force on an object measured in newtons, which varies with gravitational field strength. An astronaut has the same mass (and same inertia) on the Moon as on Earth, even though their weight is only 1/6th as large.
Exam tip: If a question asks you to compare the inertia of two objects, ignore any information about their speed, weight, or location, and only compare their masses. Inertia depends exclusively on inertial mass.
2. Inertial vs Non-Inertial Reference Frames★★★☆☆⏱ 3 min
Newton’s First Law only holds in inertial reference frames, so distinguishing between the two frame types is a key AP Physics 1 skill. A reference frame is the coordinate system you use to measure position and velocity of objects.
In non-inertial frames, you can observe objects accelerating even when the net real force on them is zero, which violates Newton’s First Law. Apparent "forces" you feel in accelerating frames (like being pushed back when a car accelerates) are fictitious forces: they are not real interaction forces, just effects of your inertia in an accelerating frame.
Exam tip: Never assume a frame is inertial just because you are observing an object at rest relative to it. Always check if the frame itself is accelerating first.
3. Translational Equilibrium★★★☆☆⏱ 5 min
The most common application of Newton’s First Law on the AP Physics 1 exam is analysis of objects in translational equilibrium. By definition, an object is in translational equilibrium if and only if the net external force on the object is zero, which per Newton’s First Law means the object’s velocity is constant. Equilibrium has two subcategories: static equilibrium (object at rest, $v=0$) and dynamic equilibrium (object moving at constant speed in a straight line).
\sum F_x = 0 \quad \text{and} \quad \sum F_y = 0
A common misconception is that equilibrium means no forces act on the object; this is incorrect: multiple forces act, but their vector sum adds up to zero, so no acceleration. To solve equilibrium problems, you first draw a correct free-body diagram, choose a coordinate system, resolve all forces into components, then set the sum of forces on each axis to zero and solve for the unknown.
Exam tip: When solving for tension in symmetric equilibrium problems (like two identical ropes supporting a weight), always use symmetry to simplify the component equations before solving, this saves time on the exam.
4. AP-Style Concept Check★★★☆☆⏱ 2 min
Common Pitfalls
Why: Students confuse the property of matter with an interaction force, because everyday experience with friction creates the misconception that 'something has to keep things moving'.
Why: Students use the object's rest relative to the frame to conclude zero net force, forgetting the frame itself is accelerating.
Why: Everyday experience with friction and air resistance leads students to think you need to push to keep moving, forgetting the push is just balancing friction to get zero net force.
Why: Students think heavier (by weight) or faster moving objects have more inertia, confusing momentum (which depends on speed) with inertia.
Why: Students misremember the rule that net force is zero as each individual force being equal.