Physics (US High School)
Comprehensive deck covering US High School Physics (NGSS-aligned, non-AP): kinematics, dynamics, energy, momentum, circular motion, gravitation, waves, sound, optics, electricity, magnetism, and an introduction to modern physics.
Ämne: Fysik · Nivå: Gymnasium (16–19) · 480 kort
Innehåll
- Displacement is a vector quantity representing the change in position: Δx = x_final − x_initial. Unlike distance, displacement has both magnitude and direction.
- Speed is a scalar (magnitude only); velocity is a vector (magnitude + direction). A car traveling 30 m/s east has a different velocity from one going 30 m/s west, but the same speed.
- Average velocity = Δx / Δt (displacement divided by time interval). SI units: m/s.
- Acceleration is the rate of change of velocity: a = Δv / Δt. SI units: m/s². Acceleration is a vector — it can result from change in speed, direction, or both.
- The five kinematic equations for constant acceleration: (1) v = v₀ + at, (2) Δx = v₀t + ½at², (3) v² = v₀² + 2aΔx, (4) Δx = ½(v₀+v)t, (5) Δx = vt − ½at².
- Near Earth's surface, the acceleration due to gravity is g ≈ 9.8 m/s² (often rounded to 10 m/s²), directed downward. This value is independent of an object's mass when air resistance is ignored.
- In free fall (ignoring air resistance), all objects fall with the same acceleration g, regardless of mass. A feather and a hammer dropped in vacuum hit the ground simultaneously — demonstrated on the Moon by Apollo 15.
- Projectile motion can be analyzed by separating into independent horizontal (constant velocity) and vertical (constant acceleration g) components. The two motions share only the time variable.
- For projectiles launched and landing at the same height, the maximum range occurs at a 45° launch angle. Complementary angles (e.g., 30° and 60°) give the same range.
- On a position-time (x-t) graph, the slope at any point equals the instantaneous velocity. A straight line means constant velocity; a curved line means acceleration.
- On a velocity-time (v-t) graph, the slope equals acceleration and the area under the curve equals displacement.
- A reference frame is a coordinate system from which motion is described. Motion is always relative — a passenger on a moving train is at rest relative to the train but moving relative to the ground.
- Vector addition: vectors can be added graphically (tip-to-tail) or by adding components. Two perpendicular components give a resultant of magnitude √(A_x² + A_y²).
- Vector components: a vector at angle θ from the x-axis has components A_x = A·cosθ and A_y = A·sinθ.
- Instantaneous velocity is the velocity at a single moment, found as the limit of Δx/Δt as Δt approaches zero. Geometrically, it is the slope of the tangent line on a position-time graph.
- Newton's First Law (law of inertia): an object at rest remains at rest, and an object in motion continues at constant velocity, unless acted on by a net external force.
- Newton's Second Law: F_net = ma. The net force on an object equals its mass times its acceleration. F is in newtons (N), m in kg, a in m/s². 1 N = 1 kg·m/s².
- Newton's Third Law: for every action force there is an equal and opposite reaction force. The two forces act on different objects — never the same object.
- Inertia is the tendency of an object to resist changes in motion. Mass is the quantitative measure of inertia — the more mass, the more force needed to change its velocity.
- Weight is the gravitational force on an object: W = mg. On Earth, a 1 kg mass weighs about 9.8 N. Mass stays constant; weight changes with gravitational field strength.
- The normal force (F_N) is the support force perpendicular to a surface. On a flat horizontal surface with no vertical acceleration, F_N = mg. On an incline at angle θ, F_N = mg·cosθ.
- Static friction (f_s) prevents motion up to a maximum value: f_s ≤ μ_s·F_N. Kinetic friction acts on a moving object: f_k = μ_k·F_N. Generally μ_s > μ_k.
- Tension is the pulling force transmitted through a rope, string, or cable. In an ideal (massless, inextensible) rope, tension has the same magnitude throughout.
- A free-body diagram shows only the forces acting ON a single object, drawn as arrows from a point representing the object. It is the first step in most Newton's-Law problems.
- An object is in translational equilibrium when the net force on it is zero. By Newton's First Law, such an object is either at rest or moving with constant velocity.
- On a frictionless incline at angle θ, an object slides down with acceleration a = g·sinθ. The component of gravity along the incline causes the acceleration.
- Terminal velocity is the constant speed reached when air resistance (drag) equals gravity. A skydiver in spread position reaches ≈ 53 m/s; head-down it can exceed 90 m/s.
- Apparent weight is what a scale reads. In an elevator accelerating upward at a, apparent weight = m(g+a). In free fall (a = −g), apparent weight is zero — the sensation of "weightlessness".
- Work done by a constant force: W = F·d·cosθ, where θ is the angle between force and displacement. SI unit: joule (J) = N·m. Only the force component along the displacement does work.
- Kinetic energy of a moving object: KE = ½·m·v². SI units: joules. KE is always positive; it doubles when mass doubles, but quadruples when speed doubles.
- Gravitational potential energy near Earth's surface: PE_grav = mgh, where h is height above a reference level. Only changes in PE matter physically; the zero level is chosen for convenience.
- Elastic potential energy stored in an ideal spring: PE_spring = ½·k·x², where k is the spring constant (N/m) and x is the displacement from the spring's natural length.
- Hooke's Law for an ideal spring: F = −k·x. The restoring force is proportional to the displacement and directed opposite to it. k is measured in N/m.
- The work-energy theorem: the net work done on an object equals its change in kinetic energy: W_net = ΔKE = ½·m·v_f² − ½·m·v_i².
- Conservation of mechanical energy (no friction or other non-conservative forces): KE_i + PE_i = KE_f + PE_f. Energy converts between forms but the total stays constant.
- Power is the rate of doing work or transferring energy: P = W/t = F·v. SI unit: watt (W) = J/s. One horsepower ≈ 746 W.
- Conservative forces (like gravity and ideal springs) do work that depends only on initial and final positions — not on path. Non-conservative forces (like friction) do path-dependent work.
- When friction is present, mechanical energy is not conserved: KE_i + PE_i = KE_f + PE_f + E_thermal. Friction converts mechanical energy to thermal energy (heat).
- Linear momentum: p = mv. Momentum is a vector with SI units kg·m/s. A bowling ball at 5 m/s has much more momentum than a baseball at the same speed.
- Impulse: J = F·Δt = Δp. The impulse on an object equals its change in momentum. SI units: N·s = kg·m/s. Same impulse can come from large force·short time or small force·long time.
- Conservation of momentum: in an isolated system (no external net force), total momentum before any interaction equals total momentum after. p_total,before = p_total,after.
- An elastic collision conserves both momentum and kinetic energy. Billiard-ball collisions and atomic collisions approximate this. Two equal masses, one stationary: incoming object stops, target leaves with original velocity.
- An inelastic collision conserves momentum but not kinetic energy — some KE becomes heat, sound, or deformation. A perfectly inelastic collision has the objects stick together afterward.
- Airbags and crumple zones extend the collision time Δt, which reduces the average force on occupants for the same change in momentum (F = Δp/Δt).
- Recoil follows from momentum conservation: a stationary cannon firing a shell of mass m at velocity v gains a backward velocity V = −mv/M, where M is the cannon's mass.
- Uniform circular motion: an object moves in a circle at constant speed. Its velocity vector is constantly changing direction, so it has a non-zero acceleration despite constant speed.
- Centripetal acceleration: a_c = v²/r, directed toward the center of the circle. A larger speed or smaller radius gives a larger centripetal acceleration.
- Centripetal force: F_c = m·v²/r, directed toward the center. It is not a new kind of force — it is whatever real force (tension, gravity, normal, friction) keeps the object in circular motion.
- There is no "centrifugal force" in an inertial frame. The outward feeling in a car turning is inertia (Newton's First Law) — your body wants to go straight while the car turns inward.
- The period T is the time for one complete revolution. Frequency f = 1/T is revolutions per second (hertz). For uniform circular motion of radius r, speed v = 2πr/T.