Newton’s first law: if the net force on a system is zero, its velocity stays constant—that means constant speed and direction (could be at rest). “Net force” means the vector sum of all forces (balanced forces → translational equilibrium, ΣF = 0). In practice: draw a free-body diagram, add force vectors; if they cancel, no acceleration. If forces are balanced in one direction but not another, the velocity changes only along the unbalanced direction. Why it “works”: objects resist changes in motion (inertia). A nonzero net force produces acceleration via Newton’s second law; zero net force produces zero acceleration, so velocity is unchanged. Also note this law holds in inertial reference frames (frames where a free object moves with constant velocity). For AP review, study Topic 2.4 (CED 2.4.A) and practice drawing FBDs and checking ΣF = 0 (Fiveable study guide: https://library.fiveable.me/ap-physics-1-revised/unit-2/4-newtons-first-law/study-guide/8pzHm6MzlRuJS1lr; unit overview: https://library.fiveable.me/ap-physics-1-revised/unit-2). For extra practice, try problems at (https://library.fiveable.me/practice/ap-physics-1-revised).

Because of inertia and Newton’s first law: if the net force on a system is zero, its velocity stays constant. Friction is just one force that can act to change velocity; if you remove friction (and there are no other unbalanced forces), the vector sum of all forces is zero (translational equilibrium) so there’s nothing to change the object’s speed or direction. Practically, that’s why a puck on near-frictionless ice keeps sliding at the same speed until a force (push, friction, bump) acts on it. Key AP terms: net force = vector sum of forces, translational equilibrium (ΣF = 0), inertia, inertial reference frame. On the exam you may be asked to identify forces on a free-body diagram or justify constant velocity by showing ΣF = 0 (CED 2.4.A). For a quick refresher, check the Topic 2.4 study guide (https://library.fiveable.me/ap-physics-1-revised/unit-2/4-newtons-first-law/study-guide/8pzHm6MzlRuJS1lr) and try practice problems (https://library.fiveable.me/practice/ap-physics-1-revised).

Balanced forces mean the vector sum of all forces on a system is zero (ΣF = 0). That situation is called translational equilibrium, and by Newton’s first law the object’s velocity stays constant—if it was at rest it stays at rest; if it was moving it keeps moving with the same speed and direction. Unbalanced forces mean ΣF ≠ 0, so there’s a nonzero net force and the object’s velocity changes (it accelerates) in the direction of that net force. Remember forces are vectors: you can have balanced forces in one direction and unbalanced in another (e.g., constant horizontal speed but accelerating vertically). These ideas are exactly Topic 2.4 in the CED (net force, translational equilibrium, Newton’s first law). For a quick review, check the Topic 2.4 study guide (https://library.fiveable.me/ap-physics-1-revised/unit-2/4-newtons-first-law/study-guide/8pzHm6MzlRuJS1lr) and practice problems at (https://library.fiveable.me/practice/ap-physics-1-revised).

Net force is the vector sum of all forces acting on the object: ΣF = F1 + F2 + F3 + … (CED 2.4.A.1). Practically: draw a free-body diagram, choose axes, break angled forces into components, then add all x-components to get ΣFx and all y-components to get ΣFy. The resultant net force is the vector (ΣFx, ΣFy); its magnitude is sqrt(ΣFx^2 + ΣFy^2) and direction arctan(ΣFy/ΣFx). If ΣF = 0 (translational equilibrium, CED 2.4.A.2) the velocity stays constant (Newton’s First Law, 2.4.A.3). Remember forces can be balanced in one direction but unbalanced in another (2.4.A.4)—only the unbalanced direction produces acceleration. Practice by sketching free-body diagrams and solving component sums; Fiveable’s Topic 2.4 study guide (https://library.fiveable.me/ap-physics-1-revised/unit-2/4-newtons-first-law/study-guide/8pzHm6MzlRuJS1lr) and the Unit 2 page (https://library.fiveable.me/ap-physics-1-revised/unit-2) have worked examples and practice questions (https://library.fiveable.me/practice/ap-physics-1-revised) that match AP exam style.

Short answer: no—they’re related but not the same. Inertia is a property of matter: an object’s tendency to keep its velocity (speed and direction) unless a net force acts on it. Newton’s First Law is the statement that describes what happens because of inertia: if the net force on a system is zero (translational equilibrium), its velocity stays constant (CED 2.4.A.3 and 2.4.A.2). So inertia explains why the First Law is true for objects with mass. A couple quick clarifications useful for the AP exam: - “Net force = 0 → constant velocity” is the law you’ll apply on problems (use free-body diagrams to check balanced vs. unbalanced forces). - Inertia isn’t a force; it’s the reason an object resists changes in motion. - Newton’s First Law is valid in inertial reference frames (CED 2.4.A.5). If you want a focused review, check the Topic 2.4 study guide (https://library.fiveable.me/ap-physics-1-revised/unit-2/4-newtons-first-law/study-guide/8pzHm6MzlRuJS1lr) and practice problems for Unit 2 (https://library.fiveable.me/ap-physics-1-revised/unit-2 and https://library.fiveable.me/practice/ap-physics-1-revised).

When forces are in equilibrium it means the vector sum of all forces on a system is zero: ΣF_i = 0. That condition is called translational equilibrium (CED 2.4.A.2). In practice it means there’s no net force to change the object’s velocity, so its speed and direction stay constant (Newton’s first law, 2.4.A.3). Key points to remember: - Equilibrium is about the net force being zero, not that every force is zero. Opposite forces can cancel (balanced forces). - Forces are vectors, so you must check components: they can be balanced in x but unbalanced in y (2.4.A.4). Only unbalanced components cause acceleration. - Use free-body diagrams to write ΣF_x = 0 and ΣF_y = 0 and solve for unknowns (keywords: normal force, friction, tension). For AP prep practice, review Topic 2.4 study guide (https://library.fiveable.me/ap-physics-1-revised/unit-2/4-newtons-first-law/study-guide/8pzHm6MzlRuJS1lr) and try problems from the Unit 2 practice set (https://library.fiveable.me/practice/ap-physics-1-revised).

Yes. Forces are vectors, so you resolve them into components. If the sum of forces in one direction is zero but the sum in a perpendicular direction is nonzero, the forces are balanced in the first direction but unbalanced in the other. That’s exactly what CED Essential Knowledge 2.4.A.4 says: translational equilibrium (ΣF = 0) can hold in one dimension but not another, and the object’s velocity changes only along the direction with the unbalanced force. Example: a block sliding on a table with no vertical acceleration—vertical forces (gravity and normal) are balanced, but a net horizontal push causes horizontal acceleration. Use free-body diagrams and component sums (ΣFx, ΣFy) to check balance. For AP-style problems, always show vector components and state ΣF components = 0 (or ≠ 0) to justify constant velocity or acceleration. For a focused review, see the Topic 2.4 study guide (https://library.fiveable.me/ap-physics-1-revised/unit-2/4-newtons-first-law/study-guide/8pzHm6MzlRuJS1lr) and practice problems (https://library.fiveable.me/practice/ap-physics-1-revised).

You need inertial reference frames because Newton’s first law—“if the net force is zero, velocity stays constant”—is only true in those frames. An inertial frame is any frame that is not accelerating: either at rest or moving at constant velocity. From an inertial frame you can draw free-body diagrams, set ΣF = 0 for translational equilibrium, and correctly predict constant velocity (including v = 0). If you observe from a non-inertial (accelerating or rotating) frame you’ll see apparent (“fictitious”) forces—e.g., feeling pushed back in an accelerating car—and Newton’s first law seems violated unless you add those extra forces. For AP problems, pick an inertial frame (often the ground) so ΣF = ma and the CED statements 2.4.A.1–5 apply directly. For a refresher, check the Topic 2.4 study guide (https://library.fiveable.me/ap-physics-1-revised/unit-2/4-newtons-first-law/study-guide/8pzHm6MzlRuJS1lr), the Unit 2 overview (https://library.fiveable.me/ap-physics-1-revised/unit-2), and practice questions (https://library.fiveable.me/practice/ap-physics-1-revised).

You know the net force is zero when the vector sum of ALL forces on the system equals zero (Σ→F i = 0). Practically: - Draw a clear free-body diagram showing every force (gravity, normal, friction, tension, etc.). - Resolve forces into perpendicular components (usually x and y). - Add components: if ΣFx = 0 and ΣFy = 0, then Σ→F = 0 (translational equilibrium) and the system’s velocity is constant (a = 0) in an inertial frame. - Watch directions: forces can be balanced in one axis and unbalanced in another—only the unbalanced axis causes acceleration. Quick checks: a stationary box with N = mg has Σ→F = 0; a box sliding at constant speed with friction exactly balanced by a push also has Σ→F = 0. On the AP exam, you’ll often earn points for showing the FBD, resolving components, and writing ΣFx = 0, ΣFy = 0 (CED 2.4.A). For focused review, see the Topic 2.4 study guide (https://library.fiveable.me/ap-physics-1-revised/unit-2/4-newtons-first-law/study-guide/8pzHm6MzlRuJS1lr) and try practice problems (https://library.fiveable.me/practice/ap-physics-1-revised).

If the net force on a system is zero (translational equilibrium), the system’s velocity stays constant—both magnitude and direction. That’s Newton’s First Law: zero resultant force means zero acceleration, so v(t) doesn’t change. Remember net force is the vector sum of all forces, so forces can be “balanced” overall even if individual forces are nonzero (ΣF = 0). Also note: forces can be balanced in one direction but not another—the velocity only changes along directions with an unbalanced force. This all assumes you’re in an inertial reference frame where Newton’s laws hold. On the AP exam, you might be asked to identify equilibrium from free-body diagrams or to explain why acceleration is zero (use ΣF = 0). For a quick review, check the Topic 2.4 study guide on Fiveable (https://library.fiveable.me/ap-physics-1-revised/unit-2/4-newtons-first-law/study-guide/8pzHm6MzlRuJS1lr) and do practice problems in Unit 2 (https://library.fiveable.me/ap-physics-1-revised/unit-2).

Gravity does pull the book down (that’s the weight, mg), but the table pushes up with an equal and opposite normal force. Those two forces add as vectors and give a net force of zero in the vertical direction—so the book’s velocity stays constant (zero). That’s translational equilibrium and exactly what Newton’s First Law says: if ΣF = 0, velocity doesn’t change (CED 2.4.A.2–2.4.A.3). A quick free-body diagram helps: arrow down labeled mg, arrow up at the contact point labeled FN. If there’s no unbalanced horizontal force (and static friction just prevents sliding), the book won’t move. If you want a refresher with examples and practice, check the Topic 2.4 study guide (https://library.fiveable.me/ap-physics-1-revised/unit-2/4-newtons-first-law/study-guide/8pzHm6MzlRuJS1lr) and extra problems (https://library.fiveable.me/practice/ap-physics-1-revised).

If an object isn’t accelerating, its free-body diagram (FBD) still shows all forces—but they sum to zero. Steps: 1. Sketch the object as a dot or simple shape. Draw every force as an arrow starting on the object and pointing away (gravity mg down, normal N perpendicular to contact, tension T along ropes, friction f along surfaces). 2. Label each arrow with a symbol and direction. Don’t draw net force—draw individual forces. 3. Choose axes (often x along a surface or incline, y perpendicular). Resolve forces into components if needed. 4. Write equilibrium equations: ΣFx = 0 and ΣFy = 0 (vector sum = 0 per CED 2.4.A.1–2.4.A.3). Solve for unknowns (e.g., static friction = mg sinθ on a block at rest on an incline). 5. Check units and directions; remember forces can be balanced in one axis but not another (CED 2.4.A.4). For AP: neat, labeled FBDs + ΣF equations earn points on FRQs. For extra practice, see the Topic 2.4 study guide (https://library.fiveable.me/ap-physics-1-revised/unit-2/4-newtons-first-law/study-guide/8pzHm6MzlRuJS1lr) and unit practice problems (https://library.fiveable.me/ap-physics-1-revised/unit-2).

If an object moves at constant velocity, Newton’s first law is exactly what’s going on: the vector sum of all forces on the object equals zero (translational equilibrium), so there’s no net force to change its velocity (CED 2.4.A.2–2.4.A.3). That means speed and direction stay the same in an inertial frame—if forces are balanced in one direction but unbalanced in another, the velocity changes only along the unbalanced direction (2.4.A.4). Practically, you check a free-body diagram: if all force components cancel (ΣF = 0), then a = 0 and v is constant. This idea shows up a lot on the exam in both multiple choice and FRs for predicting motion from forces, so practice drawing FBDs and checking vector sums. For a focused review, see the Topic 2.4 study guide (https://library.fiveable.me/ap-physics-1-revised/unit-2/4-newtons-first-law/study-guide/8pzHm6MzlRuJS1lr), unit overview (https://library.fiveable.me/ap-physics-1-revised/unit-2), and hundreds of practice problems (https://library.fiveable.me/practice/ap-physics-1-revised).

Because of Newton’s first law: an object keeps moving at constant velocity only if the net force on it is zero. On ice the surface is very smooth and the kinetic friction coefficient μk is very small, so the horizontal frictional force is nearly zero—the puck’s net horizontal force ≈ 0, so its velocity stays almost constant (inertia). On concrete the microscopic roughness and larger μk produce a much larger frictional force (Ff = μkN) opposite the motion. That unbalanced force produces acceleration (deceleration), so the puck slows and stops. Air resistance is usually negligible here; the dominant difference is surface friction and contact deformation at the microscopic level. This is exactly a Topic 2.4 idea: compare net forces (translational equilibrium vs unbalanced force). For a quick review, see the Topic 2.4 study guide (https://library.fiveable.me/ap-physics-1-revised/unit-2/4-newtons-first-law/study-guide/8pzHm6MzlRuJS1lr) and more unit practice (https://library.fiveable.me/ap-physics-1-revised/unit-2).

Start with a clear free-body diagram and choose x and y axes. Remember CED 2.4.A: the net force is the vector sum of all forces, and forces can be balanced in one direction but not the other (2.4.A.4). Steps: 1. Draw FBD with all forces (gravity, normal, tension, friction). 2. Resolve any angled forces into x and y components. 3. Write ∑Fx = 0 for the balanced direction. Use that equation to solve for unknowns (e.g., friction = component of tension). 4. Write ∑Fy = may equal may not equal 0: ∑Fy = m ay. If unbalanced, set equal to m·a_y and solve for vertical acceleration or another unknown. 5. Use kinematics if asked for motion (e.g., ay gives vy(t) or y(t)). Key idea: a = 0 only in directions where ∑F = 0. If x is balanced, vx is constant; if y is unbalanced, vy changes. For more practice and CED-aligned examples, see the Topic 2.4 study guide (https://library.fiveable.me/ap-physics-1-revised/unit-2/4-newtons-first-law/study-guide/8pzHm6MzlRuJS1lr) and more practice problems (https://library.fiveable.me/practice/ap-physics-1-revised).