This question tests understanding of what causes kinetic energy to change—specifically, that work done by forces or energy conversions cause KE to increase, decrease, or remain constant. Kinetic energy changes are governed by the work-energy relationship and energy conservation: (1) KE increases when positive work is done (force in direction of motion: engine accelerating car, force forward while car moves forward, does positive work increasing KE from 0 to ½mv²), or when other energy converts to KE (falling object: PE → KE, chemical from fuel → KE in engine); (2) KE decreases when negative work is done (force opposite to motion: friction on sliding object, force backward while object moves forward, does negative work removing KE and converting to thermal), or when KE converts to other forms (rising ball: KE → PE, collision: KE → thermal/sound/deformation); and (3) KE stays constant when no net work is done (balanced forces: F_net=0 so no acceleration, speed constant means KE constant), or when motion is perpendicular to force (satellite orbit: gravity pulls inward perpendicular to velocity, no work, KE unchanged). A hockey puck gliding at constant 10 m/s on smooth ice has constant kinetic energy KE = ½m(10²) = 50m J throughout—KE doesn't change because speed doesn't change (constant v means constant v² means constant ½mv²), and speed stays constant because net force is approximately zero: on smooth ice, friction is negligible, gravity balanced by normal force (perpendicular to motion, do no work), so no horizontal forces means F_net ≈ 0, no acceleration (a=0 from F=ma with F=0), meaning velocity constant (no Δv), meaning KE constant (no ΔKE). The work-energy perspective: net work equals approximately zero (no significant forces doing work along motion direction: W_net ≈ 0), so no net work means no KE change (W_net = ΔKE ≈ 0), explaining constant energy with essentially no forces acting horizontally (Newton's First Law: object in motion stays in motion at constant velocity when F_net = 0). Choice A is correct because it properly identifies zero net work for constant KE: the puck's kinetic energy stays constant because there is (approximately) no net force doing work on it, so its speed stays constant. Choice B incorrectly claims friction is constantly adding energy, when smooth ice has negligible friction and any small friction would remove energy, not add it; choice C contradicts itself by claiming KE stays constant while speed increases, violating KE = ½mv² (if v increases, KE must increase); choice D falsely states gravity increases speed on level ice, when gravity acts vertically (perpendicular to horizontal motion) doing no work. Explaining KE changes systematically: (1) observe motion: speeding up (KE increasing), slowing down (KE decreasing), or constant speed (KE constant), (2) identify forces: what forces act? in what directions relative to motion?, (3) determine net work: forces in motion direction do positive work (add KE), forces opposing motion do negative work (remove KE), perpendicular forces do zero work (don't change KE), balanced forces: W_net=0 (KE constant), (4) apply work-energy: W_net = ΔKE (work equals energy change), (5) check energy conversions: if work doesn't fully explain, look for conversions (PE ↔ KE, chemical → KE, KE → thermal), and (6) verify conservation: energy source for KE increase? (work input, PE decreasing, chemical burning), energy destination for KE decrease? (work output, PE increasing, thermal from friction)—all energy accounted. Common scenarios: (1) free fall: KE increases (PE → KE, gravity does positive work accelerating downward), (2) thrown upward: KE decreases then increases (KE → PE going up as gravity does negative work, PE → KE coming down as gravity does positive work), (3) braking: KE decreases (friction does negative work, KE → thermal in brakes), (4) horizontal surface no friction: KE constant (no forces doing work, speed constant by Newton's First Law), (5) engine accelerating: KE increases (chemical energy → thermal → mechanical work → KE, engine does positive work).

This question tests understanding of what causes kinetic energy to change—specifically, that work done by forces or energy conversions cause KE to increase, decrease, or remain constant. Kinetic energy changes are governed by the work-energy relationship and energy conservation: (1) KE increases when positive work is done (force in direction of motion: engine accelerating car, force forward while car moves forward, does positive work increasing KE from 0 to ½mv²), or when other energy converts to KE (falling object: PE → KE, chemical from fuel → KE in engine); (2) KE decreases when negative work is done (force opposite to motion: friction on sliding object, force backward while object moves forward, does negative work removing KE and converting to thermal), or when KE converts to other forms (rising ball: KE → PE, collision: KE → thermal/sound/deformation); and (3) KE stays constant when no net work is done (balanced forces: F_net=0 so no acceleration, speed constant means KE constant), or when motion is perpendicular to force (satellite orbit: gravity pulls inward perpendicular to velocity, no work, KE unchanged). A sliding book initially moving quickly (KE_initial = ½mv₀² where v₀ is initial speed) gradually slows due to friction and stops (KE_final = 0), losing all its kinetic energy—this KE decrease occurs because friction does negative work: friction force acts backward (opposite to book's forward motion), and as book slides distance d forward, friction does work W = -f×d (negative because force opposes displacement), removing kinetic energy which converts to thermal energy (floor and book warm microscopically from friction). The work-energy theorem explains: W_friction = ΔKE = 0 - ½mv₀² = -½mv₀² (negative work equals negative KE change), and conservation requires: lost KE (½mv₀²) = thermal energy gained (floor + book warmer by ½mv₀² total), so KE didn't disappear, it converted to thermal through friction's negative work. Choice B is correct because it correctly cites negative work and energy conversion: friction acts opposite the motion, so it does negative work and converts some of the book's kinetic energy into thermal energy. Choice A reverses work direction by claiming friction acts in same direction as motion and does positive work, when friction always opposes motion and does negative work; choice C nonsensically states KE converts into more KE; choice D falsely claims mass becomes smaller, when book's mass remains constant throughout sliding. Explaining KE changes systematically: (1) observe motion: speeding up (KE increasing), slowing down (KE decreasing), or constant speed (KE constant), (2) identify forces: what forces act? in what directions relative to motion?, (3) determine net work: forces in motion direction do positive work (add KE), forces opposing motion do negative work (remove KE), perpendicular forces do zero work (don't change KE), balanced forces: W_net=0 (KE constant), (4) apply work-energy: W_net = ΔKE (work equals energy change), (5) check energy conversions: if work doesn't fully explain, look for conversions (PE ↔ KE, chemical → KE, KE → thermal), and (6) verify conservation: energy source for KE increase? (work input, PE decreasing, chemical burning), energy destination for KE decrease? (work output, PE increasing, thermal from friction)—all energy accounted. Understanding causes of KE changes helps: predict motion (if force forward, object will speed up increasing KE), explain energy (where did KE come from? work or conversion; where did it go? work or conversion), design systems (want high KE? apply large force over distance doing lots of work; want to reduce KE? use friction or convert to other form), and reason about safety (high-speed objects have large KE requiring large work/distance to stop—why speed limits matter, why braking distances long at high speeds).

This question tests understanding of what causes kinetic energy to change—specifically, that work done by forces or energy conversions cause KE to increase, decrease, or remain constant. Kinetic energy changes are governed by the work-energy relationship and energy conservation: (1) KE increases when positive work is done (force in direction of motion: engine accelerating car, force forward while car moves forward, does positive work increasing KE from 0 to ½mv²), or when other energy converts to KE (falling object: PE → KE, chemical from fuel → KE in engine); (2) KE decreases when negative work is done (force opposite to motion: friction on sliding object, force backward while object moves forward, does negative work removing KE and converting to thermal), or when KE converts to other forms (rising ball: KE → PE, collision: KE → thermal/sound/deformation); and (3) KE stays constant when no net work is done (balanced forces: F_net=0 so no acceleration, speed constant means KE constant), or when motion is perpendicular to force (satellite orbit: gravity pulls inward perpendicular to velocity, no work, KE unchanged). A cyclist moving forward who brakes to a stop experiences decreasing kinetic energy from KE_initial = ½mv² (moving) to KE_final = 0 (stopped)—this KE decrease occurs because the braking force does negative work: brake pads apply backward force (opposite to bike's forward motion), and as bike moves forward distance d while braking, brakes do work W = -F_brake×d (negative because force opposes displacement), removing kinetic energy which converts to thermal energy (brake pads and wheel rims heat up from friction). The work-energy theorem explains: W_braking = ΔKE = 0 - ½mv² = -½mv² (negative work equals negative KE change), and conservation requires: lost KE (½mv²) = thermal energy gained (brakes warmer by ½mv² total), so KE transforms to thermal through braking's negative work. Choice A is correct because it accurately explains KE decrease using negative work: the braking force does negative work on the bike (force backward, motion forward = negative work), so the bike's kinetic energy decreases as it slows. Choice B contradicts physics by claiming braking force does positive work while KE decreases, when positive work would increase KE; choice C incorrectly states forces cannot change KE, violating the work-energy theorem; choice D falsely claims braking force is in same direction as motion and does zero work, when brakes clearly oppose motion doing negative work. Explaining KE changes systematically: (1) observe motion: speeding up (KE increasing), slowing down (KE decreasing), or constant speed (KE constant), (2) identify forces: what forces act? in what directions relative to motion?, (3) determine net work: forces in motion direction do positive work (add KE), forces opposing motion do negative work (remove KE), perpendicular forces do zero work (don't change KE), balanced forces: W_net=0 (KE constant), (4) apply work-energy: W_net = ΔKE (work equals energy change), (5) check energy conversions: if work doesn't fully explain, look for conversions (PE ↔ KE, chemical → KE, KE → thermal), and (6) verify conservation: energy source for KE increase? (work input, PE decreasing, chemical burning), energy destination for KE decrease? (work output, PE increasing, thermal from friction)—all energy accounted. Understanding causes of KE changes helps: predict motion (if force forward, object will speed up increasing KE), explain energy (where did KE come from? work or conversion; where did it go? work or conversion), design systems (want high KE? apply large force over distance doing lots of work; want to reduce KE? use friction or convert to other form), and reason about safety (high-speed objects have large KE requiring large work/distance to stop—why speed limits matter, why braking distances long at high speeds).

This question tests understanding of what causes kinetic energy to change—specifically, that work done by forces or energy conversions cause KE to increase, decrease, or remain constant. Kinetic energy changes are governed by the work-energy relationship and energy conservation: (1) KE increases when positive work is done (force in direction of motion: engine accelerating car, force forward while car moves forward, does positive work increasing KE from 0 to ½mv²), or when other energy converts to KE (falling object: PE → KE, chemical from fuel → KE in engine); (2) KE decreases when negative work is done (force opposite to motion: friction on sliding object, force backward while object moves forward, does negative work removing KE and converting to thermal), or when KE converts to other forms (rising ball: KE → PE, collision: KE → thermal/sound/deformation); and (3) KE stays constant when no net work is done (balanced forces: F_net=0 so no acceleration, speed constant means KE constant), or when motion is perpendicular to force (satellite orbit: gravity pulls inward perpendicular to velocity, no work, KE unchanged). A rubber ball rolling into a pillow and stopping loses all its kinetic energy from KE_initial = ½mv² to KE_final = 0—this KE decrease occurs during the inelastic collision where kinetic energy converts to other forms: the pillow does negative work on the ball (pillow pushes backward on ball while ball moves forward into it), and this work removes KE which transforms into (1) thermal energy (ball and pillow molecules vibrate more, warming slightly), (2) sound energy (thud of impact), and (3) deformation energy (pillow compresses, ball deforms temporarily). Conservation of energy requires: initial KE = thermal + sound + deformation + any remaining KE, but since ball stops, all initial KE converted to other forms (no KE remains), explaining complete energy transformation during collision. Choice C is correct because it properly identifies energy conversion during collision: the ball's kinetic energy decreased because some of it was converted into other forms of energy like sound, thermal energy, and deformation during the inelastic collision. Choice A incorrectly claims KE stayed same but was hidden in pillow, violating the definition of KE = ½mv² (stopped ball has v=0, so KE=0); choice B wrongly states KE increased when ball clearly stopped (KE went to zero); choice D falsely claims mass became zero, when ball's mass remains constant (only velocity became zero). Explaining KE changes systematically: (1) observe motion: speeding up (KE increasing), slowing down (KE decreasing), or constant speed (KE constant), (2) identify forces: what forces act? in what directions relative to motion?, (3) determine net work: forces in motion direction do positive work (add KE), forces opposing motion do negative work (remove KE), perpendicular forces do zero work (don't change KE), balanced forces: W_net=0 (KE constant), (4) apply work-energy: W_net = ΔKE (work equals energy change), (5) check energy conversions: if work doesn't fully explain, look for conversions (PE ↔ KE, chemical → KE, KE → thermal), and (6) verify conservation: energy source for KE increase? (work input, PE decreasing, chemical burning), energy destination for KE decrease? (work output, PE increasing, thermal from friction)—all energy accounted. Common scenarios: (1) free fall: KE increases (PE → KE, gravity does positive work accelerating downward), (2) thrown upward: KE decreases then increases (KE → PE going up as gravity does negative work, PE → KE coming down as gravity does positive work), (3) braking: KE decreases (friction does negative work, KE → thermal in brakes), (4) horizontal surface no friction: KE constant (no forces doing work, speed constant by Newton's First Law), (5) engine accelerating: KE increases (chemical energy → thermal → mechanical work → KE, engine does positive work).

This question tests understanding of what causes kinetic energy to change—specifically, that work done by forces or energy conversions cause KE to increase, decrease, or remain constant. Kinetic energy changes are governed by the work-energy relationship and energy conservation: (1) KE increases when positive work is done (force in direction of motion: engine accelerating car, force forward while car moves forward, does positive work increasing KE from 0 to ½mv²), or when other energy converts to KE (falling object: PE → KE, chemical from fuel → KE in engine); (2) KE decreases when negative work is done (force opposite to motion: friction on sliding object, force backward while object moves forward, does negative work removing KE and converting to thermal), or when KE converts to other forms (rising ball: KE → PE, collision: KE → thermal/sound/deformation); and (3) KE stays constant when no net work is done (balanced forces: F_net=0 so no acceleration, speed constant means KE constant), or when motion is perpendicular to force (satellite orbit: gravity pulls inward perpendicular to velocity, no work, KE unchanged). When the student pulls the wagon forward with constant force and the wagon speeds up from slow to fast walk, kinetic energy increases from KE_initial = ½m(v_slow)² to KE_final = ½m(v_fast)² where v_fast > v_slow—this KE increase results from the student doing positive work on the wagon: the student applies forward force (pulling the handle forward), wagon moves forward (travels distance while accelerating), and work = force × distance in the direction of force is positive (both force and displacement point forward), with this work equal to the KE gained (work-energy theorem: W = ΔKE = ½m(v_fast)² - ½m(v_slow)²). Choice A is correct because it accurately explains KE increase using positive work: the forward pulling force (same direction as motion) does positive work over the distance traveled, and this positive work increases the wagon's kinetic energy as shown by its increasing speed. Choice B reverses work direction: claims forward force does negative work, when forces in the direction of motion always do positive work (negative work requires force opposing motion); Choice C claims net force is zero, which would mean no acceleration and constant speed (Newton's Second Law: F_net = 0 means a = 0), contradicting the stated speed increase; Choice D violates conservation: suggests energy is created from nothing, when energy must come from work done by the student (chemical energy in muscles → mechanical work → wagon's KE). Explaining KE changes systematically: (1) observe motion: wagon speeding up from slow to fast (KE increasing), (2) identify forces: student pulls forward, friction/air resistance backward (but pull > resistance since accelerating), (3) determine net work: forward pull does positive work, resistance does negative work, but net work positive since net force forward, (4) apply work-energy: W_net = ΔKE > 0 (positive net work equals positive energy change), (5) check energy conversions: student's chemical energy (food/ATP) → mechanical work by muscles → kinetic energy of wagon, and (6) verify conservation: energy source for KE increase is student's metabolic energy converted through muscular work—all energy accounted. Understanding causes of KE changes helps: predict motion (forward force will accelerate wagon increasing its KE), explain energy flow (student's biological energy → wagon's mechanical energy through work), design systems (want faster wagon? apply larger force or pull longer distance for more work), and reason about effort (speeding up requires positive work, which requires energy expenditure by the puller—why pulling gets tiring, especially when accelerating heavy objects to high speeds).

This question tests understanding of what causes kinetic energy to change—specifically, that work done by forces or energy conversions cause KE to increase, decrease, or remain constant. Kinetic energy changes are governed by the work-energy relationship and energy conservation: (1) KE increases when positive work is done (force in direction of motion: engine accelerating car, force forward while car moves forward, does positive work increasing KE from 0 to ½mv²), or when other energy converts to KE (falling object: PE → KE, chemical from fuel → KE in engine); (2) KE decreases when negative work is done (force opposite to motion: friction on sliding object, force backward while object moves forward, does negative work removing KE and converting to thermal), or when KE converts to other forms (rising ball: KE → PE, collision: KE → thermal/sound/deformation); and (3) KE stays constant when no net work is done (balanced forces: F_net=0 so no acceleration, speed constant means KE constant), or when motion is perpendicular to force (satellite orbit: gravity pulls inward perpendicular to velocity, no work, KE unchanged). During the pushing phase, the cart speeds up so kinetic energy increases from low initial KE to higher final KE—this KE increase results from the student doing positive work on the cart: student applies forward force (push), cart moves forward (same direction as force), and work = force × distance is positive (force and displacement aligned), with this work equal to the KE gained; after pushing stops, friction does negative work (force backward, motion forward), causing KE to decrease as cart slows. The energy flow: while pushing, student's chemical energy (muscles) → mechanical work → cart's KE (increasing); after pushing, cart's KE → thermal energy via friction (decreasing KE, warming floor/wheels). Choice A is correct because it accurately explains KE increase during the correct phase: it increases while the student is pushing, because the applied force does positive work on the cart and increases its speed. Choice B wrongly claims KE increases after pushing stops, when friction actually does negative work decreasing KE; choice C incorrectly states friction increases KE, when friction always does negative work removing KE; choice D falsely claims KE cannot change unless mass changes, contradicting KE = ½mv² where v changes while m stays constant. Explaining KE changes systematically: (1) observe motion: speeding up (KE increasing), slowing down (KE decreasing), or constant speed (KE constant), (2) identify forces: what forces act? in what directions relative to motion?, (3) determine net work: forces in motion direction do positive work (add KE), forces opposing motion do negative work (remove KE), perpendicular forces do zero work (don't change KE), balanced forces: W_net=0 (KE constant), (4) apply work-energy: W_net = ΔKE (work equals energy change), (5) check energy conversions: if work doesn't fully explain, look for conversions (PE ↔ KE, chemical → KE, KE → thermal), and (6) verify conservation: energy source for KE increase? (work input, PE decreasing, chemical burning), energy destination for KE decrease? (work output, PE increasing, thermal from friction)—all energy accounted. Understanding causes of KE changes helps: predict motion (if force forward, object will speed up increasing KE), explain energy (where did KE come from? work or conversion; where did it go? work or conversion), design systems (want high KE? apply large force over distance doing lots of work; want to reduce KE? use friction or convert to other form), and reason about safety (high-speed objects have large KE requiring large work/distance to stop—why speed limits matter, why braking distances long at high speeds).

This question tests understanding of what causes kinetic energy to change—specifically, that work done by forces or energy conversions cause KE to increase, decrease, or remain constant. Kinetic energy changes are governed by the work-energy relationship and energy conservation: (1) KE increases when positive work is done (force in direction of motion: engine accelerating car, force forward while car moves forward, does positive work increasing KE from 0 to ½mv²), or when other energy converts to KE (falling object: PE → KE, chemical from fuel → KE in engine); (2) KE decreases when negative work is done (force opposite to motion: friction on sliding object, force backward while object moves forward, does negative work removing KE and converting to thermal), or when KE converts to other forms (rising ball: KE → PE, collision: KE → thermal/sound/deformation); and (3) KE stays constant when no net work is done (balanced forces: F_net=0 so no acceleration, speed constant means KE constant), or when motion is perpendicular to force (satellite orbit: gravity pulls inward perpendicular to velocity, no work, KE unchanged). For a ball rolling down a ramp from rest, halfway down it speeds up (KE increases from 0 to ½mv²)—this KE increase results from gravity doing positive work: component of gravity force along ramp acts downward along motion, ball moves down ramp distance d, work = (mg sinθ) × d (positive, force and displacement same direction), with this work equal to KE gained (W = ΔKE). The energy source is gravitational potential energy (PE decreases as height drops), converting PE → KE, so conservation: ΔPE = -mgΔh = ΔKE = ½mv² (lost PE becomes gained KE, ignoring friction). Choice B is correct because it accurately explains KE increase using positive work or energy conversion to KE from PE. Choice A is wrong because it misidentifies energy conversion: states KE → PE when falling (actually PE → KE), and claims KE decreases as ball goes down, contradicting speeding up means KE increases. Explaining KE changes systematically: (1) observe motion: speeding up (KE increasing), slowing down (KE decreasing), or constant speed (KE constant), (2) identify forces: what forces act? in what directions relative to motion?, (3) determine net work: forces in motion direction do positive work (add KE), forces opposing motion do negative work (remove KE), perpendicular forces do zero work (don't change KE), balanced forces: W_net=0 (KE constant), (4) apply work-energy: W_net = ΔKE (work equals energy change), (5) check energy conversions: if work doesn't fully explain, look for conversions (PE ↔ KE, chemical → KE, KE → thermal), and (6) verify conservation: energy source for KE increase? (work input, PE decreasing, chemical burning), energy destination for KE decrease? (work output, PE increasing, thermal from friction)—all energy accounted. Common scenarios: (1) free fall: KE increases (PE → KE, gravity does positive work accelerating downward), (2) thrown upward: KE decreases then increases (KE → PE going up as gravity does negative work, PE → KE coming down as gravity does positive work), (3) braking: KE decreases (friction does negative work, KE → thermal in brakes), (4) horizontal surface no friction: KE constant (no forces doing work, speed constant by Newton's First Law), (5) engine accelerating: KE increases (chemical energy → thermal → mechanical work → KE, engine does positive work).

This question tests understanding of what causes kinetic energy to change—specifically, that work done by forces or energy conversions cause KE to increase, decrease, or remain constant. Kinetic energy changes are governed by the work-energy relationship and energy conservation: (1) KE increases when positive work is done (force in direction of motion: engine accelerating car, force forward while car moves forward, does positive work increasing KE from 0 to ½mv²), or when other energy converts to KE (falling object: PE → KE, chemical from fuel → KE in engine); (2) KE decreases when negative work is done (force opposite to motion: friction on sliding object, force backward while object moves forward, does negative work removing KE and converting to thermal), or when KE converts to other forms (rising ball: KE → PE, collision: KE → thermal/sound/deformation); and (3) KE stays constant when no net work is done (balanced forces: F_net=0 so no acceleration, speed constant means KE constant), or when motion is perpendicular to force (satellite orbit: gravity pulls inward perpendicular to velocity, no work, KE unchanged). After the student stops pushing, the book slows from initial speed to stop (KE decreases from ½mv² to 0)—this KE decrease occurs because friction does negative work: friction force acts backward (opposite to book's forward motion), and as book slides distance d forward, friction does work W = -f×d (negative because force opposes displacement), removing kinetic energy which converts to thermal energy (table and book warm from friction). The work-energy theorem explains: W_net = W_friction = ΔKE = 0 - ½mv² = -½mv² (negative net work equals negative KE change, since no other horizontal forces after pushing stops), and conservation requires: lost KE (½mv²) = thermal energy gained, so KE converts to thermal through friction's negative work. Choice A is correct because it correctly cites negative work or energy conversion from KE for KE decrease, connecting net force (friction opposite motion) to negative net work and KE decrease. Choice C is wrong because it reverses work direction: claims friction does positive work increasing KE, when friction does negative work decreasing KE (force opposes motion). Explaining KE changes systematically: (1) observe motion: speeding up (KE increasing), slowing down (KE decreasing), or constant speed (KE constant), (2) identify forces: what forces act? in what directions relative to motion?, (3) determine net work: forces in motion direction do positive work (add KE), forces opposing motion do negative work (remove KE), perpendicular forces do zero work (don't change KE), balanced forces: W_net=0 (KE constant), (4) apply work-energy: W_net = ΔKE (work equals energy change), (5) check energy conversions: if work doesn't fully explain, look for conversions (PE ↔ KE, chemical → KE, KE → thermal), and (6) verify conservation: energy source for KE increase? (work input, PE decreasing, chemical burning), energy destination for KE decrease? (work output, PE increasing, thermal from friction)—all energy accounted. Common scenarios: (1) free fall: KE increases (PE → KE, gravity does positive work accelerating downward), (2) thrown upward: KE decreases then increases (KE → PE going up as gravity does negative work, PE → KE coming down as gravity does positive work), (3) braking: KE decreases (friction does negative work, KE → thermal in brakes), (4) horizontal surface no friction: KE constant (no forces doing work, speed constant by Newton's First Law), (5) engine accelerating: KE increases (chemical energy → thermal → mechanical work → KE, engine does positive work).

This question tests understanding of what causes kinetic energy to change—specifically, that work done by forces or energy conversions cause KE to increase, decrease, or remain constant. Kinetic energy changes are governed by the work-energy relationship and energy conservation: (1) KE increases when positive work is done (force in direction of motion: engine accelerating car, force forward while car moves forward, does positive work increasing KE from 0 to ½mv²), or when other energy converts to KE (falling object: PE → KE, chemical from fuel → KE in engine); (2) KE decreases when negative work is done (force opposite to motion: friction on sliding object, force backward while object moves forward, does negative work removing KE and converting to thermal), or when KE converts to other forms (rising ball: KE → PE, collision: KE → thermal/sound/deformation); and (3) KE stays constant when no net work is done (balanced forces: F_net=0 so no acceleration, speed constant means KE constant), or when motion is perpendicular to force (satellite orbit: gravity pulls inward perpendicular to velocity, no work, KE unchanged). When a cyclist brakes and slows from 12 m/s to 6 m/s, kinetic energy decreases from KE_initial = ½m(12²) = 72m J to KE_final = ½m(6²) = 18m J (ΔKE = -54m J)—this KE decrease occurs because braking friction does negative work: brake force acts backward (opposite to forward motion), and as bike moves forward distance d, brakes do work W = -f×d (negative, force opposes displacement), removing kinetic energy which converts to thermal energy (brakes and tires heat up from friction). The work-energy theorem explains: W_brakes = ΔKE = 18m - 72m = -54m J (negative work equals negative KE change), and conservation requires: lost KE (54m J) = thermal energy gained (brakes warmer by 54m J), so KE converted to thermal through brakes' negative work. Choice B is correct because it correctly cites negative work or energy conversion from KE for KE decrease. Choice A is wrong because it misidentifies energy conversion: claims heat adds KE, but actually KE → thermal (heat is the destination, not source; braking removes KE to produce heat). Explaining KE changes systematically: (1) observe motion: speeding up (KE increasing), slowing down (KE decreasing), or constant speed (KE constant), (2) identify forces: what forces act? in what directions relative to motion?, (3) determine net work: forces in motion direction do positive work (add KE), forces opposing motion do negative work (remove KE), perpendicular forces do zero work (don't change KE), balanced forces: W_net=0 (KE constant), (4) apply work-energy: W_net = ΔKE (work equals energy change), (5) check energy conversions: if work doesn't fully explain, look for conversions (PE ↔ KE, chemical → KE, KE → thermal), and (6) verify conservation: energy source for KE increase? (work input, PE decreasing, chemical burning), energy destination for KE decrease? (work output, PE increasing, thermal from friction)—all energy accounted. Common scenarios: (1) free fall: KE increases (PE → KE, gravity does positive work accelerating downward), (2) thrown upward: KE decreases then increases (KE → PE going up as gravity does negative work, PE → KE coming down as gravity does positive work), (3) braking: KE decreases (friction does negative work, KE → thermal in brakes), (4) horizontal surface no friction: KE constant (no forces doing work, speed constant by Newton's First Law), (5) engine accelerating: KE increases (chemical energy → thermal → mechanical work → KE, engine does positive work).

This question tests understanding of what causes kinetic energy to change—specifically, that work done by forces or energy conversions cause KE to increase, decrease, or remain constant. Kinetic energy changes are governed by the work-energy relationship and energy conservation: (1) KE increases when positive work is done (force in direction of motion: engine accelerating car, force forward while car moves forward, does positive work increasing KE from 0 to ½mv²), or when other energy converts to KE (falling object: PE → KE, chemical from fuel → KE in engine); (2) KE decreases when negative work is done (force opposite to motion: friction on sliding object, force backward while object moves forward, does negative work removing KE and converting to thermal), or when KE converts to other forms (rising ball: KE → PE, collision: KE → thermal/sound/deformation); and (3) KE stays constant when no net work is done (balanced forces: F_net=0 so no acceleration, speed constant means KE constant), or when motion is perpendicular to force (satellite orbit: gravity pulls inward perpendicular to velocity, no work, KE unchanged). For KE decreasing: The first puck, say initially at 5 m/s (KE_initial = ½m(5²) = 12.5m J), slows to 3 m/s after collision (KE_final = ½m(3²) = 4.5m J), decreasing by 8m J—this KE decrease occurs because during collision, the contact force from the second puck acts backward on the first (opposing its motion), doing negative work, transferring some KE to the second puck's motion and possibly converting some to sound/heat/deformation if inelastic. Choice A is correct because it correctly cites energy conversion from KE for KE decrease. Choice B is wrong because it violates conservation: claims KE is destroyed, but energy is conserved, just transferred or converted to other forms like KE of second puck or thermal. Explaining KE changes systematically: (1) observe motion: speeding up (KE increasing), slowing down (KE decreasing), or constant speed (KE constant), (2) identify forces: what forces act? in what directions relative to motion?, (3) determine net work: forces in motion direction do positive work (add KE), forces opposing motion do negative work (remove KE), perpendicular forces do zero work (don't change KE), balanced forces: W_net=0 (KE constant), (4) apply work-energy: W_net = ΔKE (work equals energy change), (5) check energy conversions: if work doesn't fully explain, look for conversions (PE ↔ KE, chemical → KE, KE → thermal), and (6) verify conservation: energy source for KE increase? (work input, PE decreasing, chemical burning), energy destination for KE decrease? (work output, PE increasing, thermal from friction)—all energy accounted. Common scenarios: (1) free fall: KE increases (PE → KE, gravity does positive work accelerating downward), (2) thrown upward: KE decreases then increases (KE → PE going up as gravity does negative work, PE → KE coming down as gravity does positive work), (3) braking: KE decreases (friction does negative work, KE → thermal in brakes), (4) horizontal surface no friction: KE constant (no forces doing work, speed constant by Newton's First Law), (5) engine accelerating: KE increases (chemical energy → thermal → mechanical work → KE, engine does positive work).