AP Physics C · Unit 2
Newton's Laws of Motion: every key term you need (+ practice quiz)
36 flashcard terms for AP Physics C Unit 2, written to match the course framework. Read them here, drill them as flashcards, or take the 22-question quiz. Free, no account needed.
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Newton's First Law An object at rest stays at rest and one in motion stays in constant-velocity motion unless a net external force acts (inertia).
Newton's Second Law F_net = ma, more generally F = dp/dt; net force equals rate of change of momentum; a vector equation applied per axis.
Newton's Third Law Forces come in equal-and-opposite pairs on different bodies: F_AB = −F_BA; the two forces never act on the same object.
Free-Body Diagram A sketch of a single object with every external force drawn as a vector from it; the basis for applying ΣF = ma.
Weight W = mg; the gravitational force on a mass; a downward force distinct from mass, which is the amount of matter.
Normal Force N; the contact force perpendicular to a surface; adjusts to prevent interpenetration and is not always equal to mg.
Tension Force transmitted along a string or rope; for an ideal massless rope over an ideal pulley the tension is the same throughout.
Static Friction f_s ≤ μ_s N; opposes impending motion up to a maximum; adjusts to match applied force until slipping begins.
Kinetic Friction f_k = μ_k N; opposes sliding motion with roughly constant magnitude; usually μ_k < μ_s.
Inclined Plane Components Resolve weight along the incline (mg sinθ) and perpendicular to it (mg cosθ); the normal force balances mg cosθ.
Block on a Frictionless Incline Acceleration down the slope is a = g sinθ, independent of mass; the net force is mg sinθ along the incline.
Normal Force on an Incline N = mg cosθ for a block resting on a slope of angle θ; smaller than the weight for θ > 0.
Connected Bodies Treat linked masses as a system for acceleration, then isolate one body to find the internal tension via its own free-body diagram.
Atwood Machine Two masses over a pulley: a = (m₂ − m₁)g/(m₁ + m₂); tension T = 2m₁m₂g/(m₁ + m₂) for an ideal setup.
Resistive (Drag) Force A velocity-dependent retarding force; common models are F = −bv (linear) or F = −cv² (quadratic) opposing motion.
Equation of Motion with Linear Drag m dv/dt = mg − bv for a falling body; a first-order ODE whose solution approaches a constant speed.
Terminal Velocity The speed at which drag balances weight so net force and acceleration are zero; v_t = mg/b (linear) or √(mg/c) (quadratic).
Velocity vs Time with Drag v(t) = v_t(1 − e^(−bt/m)) for a body released from rest with linear drag; approaches v_t exponentially.
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Equilibrium When F_net = 0 the object has zero acceleration: it is at rest or moving at constant velocity (dynamic equilibrium).
Apparent Weight The normal force felt in an accelerating frame, e.g. N = m(g + a) in an upward-accelerating elevator; differs from true weight mg.
Mass vs Weight Mass (kg) is an intrinsic scalar measuring inertia; weight (N) is the gravitational force mg and varies with local g.
Newton's Second Law (General) ΣF = dp/dt; reduces to ΣF = ma only when mass is constant.
Velocity-Dependent Drag F_drag = −bv (linear) or −cv² (quadratic); leads to differential equations solved by separation of variables.
Terminal Velocity (Quadratic Drag) Setting mg = cv_T² gives v_T = √(mg/c); heavier objects of the same shape fall faster in air.
Static Friction Inequality f_s ≤ μ_sN; static friction takes whatever value prevents sliding, up to its maximum μ_sN.
Banked Curve (No Friction) tanθ = v²/(rg); the horizontal component of the normal force supplies the centripetal force.
Banked Curve with Friction Friction acts down the slope when speed exceeds the design speed and up the slope when below it.
Conical Pendulum T cosθ = mg and T sinθ = mv²/r; period is 2π√(L cosθ/g).
Vertical Circle Minimum Speed At the top of a loop, N ≥ 0 requires v ≥ √(rg); the string or track just goes slack at that speed.
Apparent Weight The normal force reading; N = m(g + a) in an accelerating elevator; zero in free fall.
Atwood Machine a = (m₂ − m₁)g/(m₁ + m₂) and T = 2m₁m₂g/(m₁ + m₂); tension is less than the heavier weight and more than the lighter.
Pulley Constraint String length is fixed, so accelerations of connected masses are related; a movable pulley halves acceleration and doubles force.
Inertial Reference Frame A frame in which Newton's first law holds; accelerating frames require fictitious (pseudo) forces.
Center of Mass Motion ΣF_ext = M a_cm; internal forces cannot accelerate the center of mass of a system.
Non-Uniform Circular Motion Tangential component a_t = dv/dt changes speed; radial component v²/r changes direction.
Free-Body Diagram Discipline Draw only forces acting on the chosen object at its center; never draw ma or 'centripetal force' as a separate force.
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