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Showing 1–30 of 1709 reference entries

Mathematics & logicExamples and counterexamples

Learn what a claim covers, explain why it works, and recognise an allowed case that shows it needs changing.

Method
Mathematics & logicCounting and zero

Count each object once, explain the final count, and distinguish quantity, position and identifying labels. Make zero’s role explicit.

Definition
Mathematics & logicNumber lines and order

Use number-line positions to compare whole numbers. Read the scale, recognise equality, and distinguish numerical distance from drawing size.

Definition
Mathematics & logicAddition and subtraction

Combine separate counts, remove a part, and find differences or missing parts. Connect objects, equations and number-line moves, then use tens and ones to regroup without changing value.

Definition
Mathematics & logicMultiplication and division

Count equal groups, distinguish sharing from making groups, and check with multiplication. Split calculations into parts; interpret zero and remainders in context.

Definition
Mathematics & logicFractions

Choose a whole, count equal fractional units, and connect sharing with division. Place fractions on the number line, including values beyond one, and explain what their numerator and denominator mean.

Definition
Mathematics & logic · Physics · MeasurementRatios and rates

Distinguish part-to-part ratios, fractions of a whole and rates with units. Scale quantities together, compare unit rates, and explain when a relationship is proportional. Check zero cases, starting amounts and whether a result fits its context.

Definition
Mathematics & logic · Physics · MeasurementQuantities and units

Describe a quantity as a numerical value with a unit, and distinguish its physical kind from its dimension. Build length, area, volume and rate units, check meaningful combinations, and separate exact unit relations from measurement uncertainty.

Definition
Mathematics & logic · Physics · MeasurementUnit conversions

Convert quantities with exact unit ratios, track cancellation through compound units, and apply the correct powers to area and volume. Distinguish temperature readings from differences, check dimensions and context, and retain measurement uncertainty when reporting.

Method
Mathematics & logic · Physics · MeasurementEstimation and uncertainty

Distinguish exact model values, rounded approximations and estimates from measurements. Declare the rounding rule, preserve information through calculations and unit changes, and interpret resolution, precision, accuracy and the stated uncertainty type without turning a report’s digits into guaranteed bounds.

Definition
Mathematics & logicVariables and expressions

Use symbols to name numbers or quantities, describe their allowed values, and evaluate expressions by replacing every occurrence consistently. Preserve grouping, signs, fractions and units, and distinguish a calculated value from solving an equation or proving two rules equivalent.

Definition
Mathematics & logicEquality and equations

Read equality as a relationship, find the permitted values that make an equation true, and justify one-step solutions by reversible operations on both sides. Work exactly with negative numbers and fractions, recognize all-solution and no-solution cases, and check the original equation, units and context.

Definition
Mathematics & logic · PhysicsCoordinates and tables

Locate a point from an ordered pair, read named axes and their scales, and connect the same values in words, tables and plots. Keep signs, fractions and units exact, distinguish an axis point from a quadrant point, and separate selected observations from a stated rule and its domain before drawing connections.

Definition
Mathematics & logic · PhysicsSlope and straight lines

Read a straight line as a stated relationship, calculate its slope from consistent signed coordinate changes, and interpret its intersections with the axes. Keep horizontal, vertical and coincident-point cases distinct, attach units and domain conditions to context models, and use numerical scales rather than appearance alone to judge a line’s change.

Definition
Mathematics & logic · Physics · MeasurementReading graphs

Read a graph’s quantities, units, ticks and context before judging its visual impression. Calculate differences, reference-based percentages and interval rates from the represented values; distinguish legitimate zoom from distorted spacing; and keep samples, interpolation, missing information, uncertainty and causal explanations separate.

Method
Physics · MeasurementObservables and measurement

Choose the physical quantity you want to investigate, specify the point, event or location involved, and explain how a reading relates to that quantity.

Method
PhysicsSystems and boundaries

Choose what belongs to an investigation, describe its condition at a stated time, and identify interactions with what lies outside its boundary.

Method
PhysicsModels and assumptions

Choose what a model represents, make its simplifications explicit, and check whether its predictions answer the physical question you actually asked.

Method
PhysicsHypotheses, laws and theories

Identify what a scientific statement does, examine its evidence and conditions, and avoid treating its name as a score for certainty.

Method
Mathematics & logic · PhysicsApproximations

Build a useful estimate, compare a correction with a stated reference scale, and check what is lost when you leave out a term.

Approximation
Mathematics & logic · PhysicsSymmetry and invariance

Specify a transformation, identify what stays unchanged, and distinguish symmetry of an object from invariance of a physical model.

Mathematical statement
PhysicsConservation laws

Use a defined system and a named quantity to track transfers, distinguish changing parts from conserved totals, and check the conditions of energy and momentum balances.

Model
PhysicsEmergence and levels of description

Choose a descriptive level, connect collective quantities with component behavior, and check when a simpler physical description answers a specified question.

Model
Mathematics & logic · PhysicsVectors

Describe displacement with fixed-axis components, combine compatible vectors, and use small exact examples to introduce span, independence, basis and real vector spaces.

Definition
Physics · Information · MeasurementScientific records

Build an inspectable physics record, test and revise a bounded cart model, preserve processing history, and plan findable data with appropriate sharing conditions.

Method
PhysicsPosition, velocity and acceleration

Describe a point's motion on a chosen straight axis, then connect signed changes, instantaneous rates and one exact set of position, velocity and acceleration graphs.

Definition
PhysicsNewton’s laws and forces

Choose one body, identify the forces acting on it, and use their vector sum to distinguish acceleration, balanced motion and mutual interactions.

Model
PhysicsWork, energy and power

Connect force and displacement to changes in motion, follow a spring’s stored energy, and distinguish an energy transfer from its rate.

Model
Physics · MeasurementSpeed of light in vacuum

c denotes the speed of light in vacuum. Its numerical value is fixed exactly in SI units.

Definition
Physics · MeasurementPlanck constant

h denotes the Planck constant. Its numerical value is fixed exactly in SI units.

Definition
ModelSource checked

Fixed-target reaction threshold from invariant energy

Physics

For projectile rest mass m_a and target m_A initially at rest, squared center-of-momentum energy is E_cm²=(m_a+m_A)²c⁴+2m_Ac²K_a.

Key knowledge

For projectile rest mass m_a and target m_A initially at rest, squared center-of-momentum energy is E_cm²=(m_a+m_A)²c⁴+2m_Ac²K_a. If the minimum allowed final invariant mass is M_f, an endothermic channel requires K_thr=[M_f²−(m_a+m_A)²]c²/(2m_A). With Q=(m_a+m_A−M_f)c² this becomes −Q(1+m_a/m_A)+Q²/(2m_Ac²), showing why laboratory threshold generally exceeds −Q.

Values and equations

E_cm²=(m_a+m_A)²c⁴+2m_Ac²K_a; K_thr=[M_f²−(m_a+m_A)²]c²/(2m_A)=−Q(1+m_a/m_A)+Q²/(2m_Ac²)

E_cm,K_a,K_thr,Q
Center-of-momentum energy, projectile kinetic/threshold energy and rest-energy release J
m_a,m_A,M_f
Projectile, target and minimum final invariant masses kg
c
Vacuum light speed m s⁻¹

Conditions and scope

  • m_A>0; isolated relativistic energy/momentum conservation, fixed target and specified final rest masses/states. For a freely separating final state with at least two massive particles, M_f is their mass sum; otherwise use the actual minimum permitted invariant energy.
  • The positive endothermic threshold is a kinematic minimum, not proof of a nonzero rate, sufficient angular-momentum/charge balance, or a universal threshold for all target geometries. The small-|Q| formula neglects the explicit Q² correction; exothermic two-or-more-body channels have no positive energy threshold from this mass condition alone.
Sources · 1

References

Sources retain their recorded scope. A source link alone does not establish verification of every statement.

  1. PDG: Kinematics (2025 edition)D. Miller, D. R. Tovey and J. D. Jackson / Particle Data GroupReference guide · PDFpdg.lbl.govRead source

Evidence and review

PDG: Kinematics (2025 edition): Sections 49.1–49.2, printed page 1, equations 49.2–49.3: invariant center-of-momentum energy and fixed-target expression in ħ=c=1 units. Threshold and Q forms are explicit algebraic derivations with c restored; no cross section is inferred.

2026-10-10 · Independent AI editorial review: All 136 unique authored Physics compact statements, all hypotheses and displayed equations/variable/units tables, all 192 Physics-origin semantic links, exact 165 author inspection matrix and mapped retained conceptual claims have been independently read. Three Physics-tagged Measurement realization records are delegated to root Measurement review. Scientific source verification combines exact author section inspection with independent whole compact-content/semantic review and targeted primary rechecks recorded below; it does not claim each full source body was independently reread. Independent AI editorial review is not human specialist certification or experimental replication. Field inventory is approved after independently reviewing the two added source-inspected band-construction methods. Receipt: physics-independent-content-inventory-review.1.

Entry detailsPermanent link to this entry ↗
Record identity
physics-reaction-threshold-kinematics
Revision
university-core.1
Structured reference record
University core expansion