This question tests understanding that non-contact forces (gravity, electric, magnetic) can do work over distances, changing potential energy even when objects aren't touching. Work is defined as force applied over distance (W = F×d, or W = mgh for lifting against gravity), and non-contact forces like gravity, electric forces, and magnetic forces can do work because they act across space without requiring contact: when you lift a book from floor to shelf, Earth's gravitational force (pulling downward at distance—book not touching Earth) acts over the 2 m height you lift through, and you do work W = mgh against this force, storing the work as gravitational potential energy (PE = mgh gained). When the magnet attracts the steel paper clip across the 5 cm air gap, the magnetic force pulls the clip toward the magnet, and since the clip moves in the direction of the magnetic force (both toward magnet), the magnetic force does positive work on the clip. This positive work by the magnetic force converts magnetic potential energy (stored in the separated configuration) into kinetic energy, causing the clip to accelerate from rest and gain speed as it approaches the magnet—demonstrating that magnetic fields can transmit force and do work across empty space without physical contact. Choice B is correct because it properly recognizes magnetic force does positive work (force and motion in same direction) which increases the clip's kinetic energy. Choice A incorrectly claims no work when the clip clearly moves through distance under magnetic force, Choice C incorrectly states negative work when force and motion are in same direction (both toward magnet), and Choice D falsely claims only contact forces can change KE when magnetic force clearly accelerates the clip across the gap. Work by non-contact forces demonstrates energy transfer without contact: gravitational force does work as objects rise or fall (lift: work against gravity → PE increases, fall: work by gravity → PE decreases, KE increases), electric forces do work as charges move in field (against force: PE increases, by force: PE decreases), and magnetic forces do work as magnets or magnetic materials move in magnetic fields (apart against attraction: PE increases, together by attraction: PE decreases)—all occurring across gaps without physical contact because fields extend through space allowing forces to act at distance.

This question tests understanding that non-contact forces (gravity, electric, magnetic) can do work over distances, changing potential energy even when objects aren't touching. Work is defined as force applied over distance (W = F×d, or W = mgh for lifting against gravity), and non-contact forces like gravity, electric forces, and magnetic forces can do work because they act across space without requiring contact: when you lift a book from floor to shelf, Earth's gravitational force (pulling downward at distance—book not touching Earth) acts over the 2 m height you lift through, and you do work W = mgh against this force, storing the work as gravitational potential energy (PE = mgh gained). When a 1 kg object is lifted from h=0 to h=3 m, gravity exerts a downward force F = mg = 1×10 = 10 N throughout the motion, while the object moves upward 3 m (opposite to gravity's direction). Since force and displacement are in opposite directions (gravity pulls down, object moves up), gravity does negative work: W = -F×d = -10 N × 3 m = -30 J (or W = -mgh = -1×10×3 = -30 J). This negative work by gravity corresponds to the positive work (+30 J) done by the lifter against gravity, and represents the 30 J of energy transferred from the lifter to gravitational PE storage. Choice B is correct because it properly calculates gravity's negative work as W = -mgh = -30 J. Choice A (+30 J) has the wrong sign (that's the lifter's work, not gravity's), Choice C (0 J) incorrectly claims no work because of non-contact when gravity clearly acts over distance, and Choice D (-3 J) appears to forget the factor of g (using just -1×3). Work by non-contact forces demonstrates energy transfer without contact: gravitational force does work as objects rise or fall (lift: work against gravity → PE increases, fall: work by gravity → PE decreases, KE increases), electric forces do work as charges move in field (against force: PE increases, by force: PE decreases), and magnetic forces do work as magnets or magnetic materials move in magnetic fields (apart against attraction: PE increases, together by attraction: PE decreases)—all occurring across gaps without physical contact because fields extend through space allowing forces to act at distance.

This question tests understanding that non-contact forces (gravity, electric, magnetic) can do work over distances, changing potential energy even when objects aren't touching. Work against gravity is W = mgh for each height change, additive for separate lifts, as gravity acts continuously. First lift (0 to 1 m): W = 5×10×1 = 50 J; second (1 to 3 m, 2 m change): W = 5×10×2 = 100 J, twice the first due to twice the height change, despite non-contact force pulling downward. Choice B is correct because it recognizes the second lift requires twice as much work against gravity due to the larger height change (2 m vs 1 m). Choice A is wrong because it claims same work, ignoring that work depends on height difference, not just mass. Work by non-contact forces demonstrates energy transfer without contact: each lift stores PE proportional to h. Systematic analysis: (1) force (gravity, down), (2) motion (up), (3) opposite, work against = mgh per segment, (4) second W=100 J > first 50 J, (5) cumulative PE increase.

This question tests understanding that non-contact forces (gravity, electric, magnetic) can do work over distances, changing potential energy even when objects aren't touching. Work is defined as W = F×d cosθ; for gravity, when lifting upward against downward force, work by gravity is negative (θ=180°, cos=-1). For the 1.5 kg bottle lifted 2 m upward at constant speed, gravity pulls downward with F=15 N, displacement upward, so W_gravity = -mg h = -1.5×10×2 = -30 J, as the non-contact force opposes the motion across the air gap. Choice B is correct because it properly calculates the negative work by gravity using W = -mgh, recognizing the sign convention for opposing force and displacement. Choice A claims +30 J, which is incorrect because it ignores the opposite directions, confusing work by gravity with work against it. Work by non-contact forces demonstrates energy transfer without contact: negative work by gravity during lift means PE increases by +30 J. Systematic analysis: (1) force (gravity, down), (2) motion (up), (3) opposite so W negative, PE increases, (4) W = -30 J, (5) energy conserved.

This question tests understanding that non-contact forces (gravity, electric, magnetic) can do work over distances, changing potential energy even when objects aren't touching. Work is defined as force applied over distance (W = F×d), and non-contact forces like gravity, electric forces, and magnetic forces can do work because they act across space without requiring contact: when you lift objects, Earth's gravitational force does work (negative work opposing the motion) even though the object never touches Earth. When lifting a 2 kg box upward 2 m at constant speed, gravitational force F = mg = 2×10 = 20 N points downward while displacement d = 2 m points upward (opposite directions), so gravity's work is W = F·d·cos(180°) = 20×2×(-1) = -40 J; the negative sign indicates gravity does negative work (opposes motion), which equals the negative of the work you do against gravity (+40 J), and represents the increase in gravitational PE. Choice B is correct because it accurately states gravity does -40 J of work, correctly recognizing that when force (downward) and displacement (upward) are opposite, work is negative: W = -mgh = -2×10×2 = -40 J. Choice A (+40 J) has wrong sign (gravity opposes upward motion, so does negative work), Choice C (0 J) incorrectly claims no work because gravity is non-contact (gravity clearly does work at distance), and Choice D (-20 J) calculates incorrectly (seems to forget height in W = mgh). Work by non-contact forces demonstrates energy transfer without contact: gravity does negative work during lifting (force opposes motion), which corresponds to the positive work you must do against gravity, storing energy as increased PE. Understanding work signs is crucial: gravity does negative work when objects move up against it (your positive work stores PE) and positive work when objects fall with it (gravity's positive work releases PE as KE).

This question tests understanding that non-contact forces (gravity, electric, magnetic) can do work over distances, changing potential energy even when objects aren't touching. Work is defined as force applied over distance (W = F×d, or W = mgh for gravity), and non-contact forces like gravity, electric forces, and magnetic forces can do work because they act across space without requiring contact: when objects fall, Earth's gravitational force (pulling downward at distance) acts over the distance fallen, and gravity does positive work W = mgh on the falling object. When a 1.5 kg ball falls 4 m, gravitational force (F = mg = 1.5×10 = 15 N pulling down) acts in the direction of motion (ball moving down), so gravity does positive work W = F×d = 15 N × 4 m = 60 J (or directly: W = mgh = 1.5×10×4 = 60 J); this positive work by gravity decreases gravitational PE by 60 J and increases kinetic energy by 60 J (ball speeds up), demonstrating that gravity's work converts PE to KE during the fall. Choice C is correct because it correctly calculates gravity's work as +60 J using W = mgh = 1.5×10×4 = 60 J, with positive sign because force and displacement are in same direction (both downward). Choice A (-60 J) has wrong sign (gravity does positive work when objects fall in its direction), Choice B (+6 J) calculates incorrectly (appears to forget factor of g), and Choice D (+15 J) seems to use just mg without multiplying by height. Work by non-contact forces demonstrates energy transfer without contact: gravitational force does positive work as objects fall (force and motion both downward), converting 60 J of gravitational PE to 60 J of KE without any contact between Earth and ball. Understanding that forces can do work at distance is essential: gravity continuously does work throughout the 4 m fall, transferring energy from the gravitational field (PE) to the ball's motion (KE).

This question tests understanding that non-contact forces (gravity, electric, magnetic) can do work over distances, changing potential energy even when objects aren't touching. Work is defined as force applied over distance (W = F×d, or W = mgh for lifting against gravity), and non-contact forces like gravity, electric forces, and magnetic forces can do work because they act across space without requiring contact: when you lift a book from floor to shelf, Earth's gravitational force (pulling downward at distance—book not touching Earth) acts over the 2 m height you lift through, and you do work W = mgh against this force, storing the work as gravitational potential energy (PE = mgh gained). Lifting a 3 kg book from floor (h=0, reference level) to a 2 m high shelf requires work against Earth's gravitational force: gravity pulls the book downward with force F = mg = 3×10 = 30 N (continuously throughout the lift, despite book not touching Earth—gravitational force acts at distance), and lifting moves the book upward through distance d = 2 m (opposite to gravitational force direction), so work done is W = F×d = 30 N × 2 m = 60 J (or directly: W = mgh = 3×10×2 = 60 J, same result). Choice B is correct because it correctly calculates work using W=mgh = 3×10×2 = 60 J, recognizing that work against gravity equals the gravitational potential energy gained. Choice A (6 J) calculates work incorrectly: appears to forget the factor of g (using just 3×2 = 6), Choice C (600 J) is off by a factor of 10 (perhaps using g=100 instead of 10), and Choice D (15 J) doesn't follow any clear formula (not mgh or any standard calculation). Work by non-contact forces demonstrates energy transfer without contact: gravitational force does work as objects rise or fall (lift: work against gravity → PE increases, fall: work by gravity → PE decreases, KE increases), and this 60 J of work done against gravity is stored as gravitational potential energy that could be recovered if the book falls back down. Understanding that forces can do work at distance (not just contact forces like friction or normal force) is essential for energy analysis: the student does 60 J of work against the non-contact gravitational force, increasing the book's gravitational potential energy by exactly 60 J.

This question tests understanding that non-contact forces (gravity, electric, magnetic) can do work over distances, changing potential energy even when objects aren't touching. Work is defined as force applied over distance (W = F×d, or W = mgh for lifting against gravity), and non-contact forces like gravity, electric forces, and magnetic forces can do work because they act across space without requiring contact: when you lift objects to different heights, Earth's gravitational force (pulling downward at distance) acts over the height you lift through, and you do work W = mgh against this force, with work proportional to height. Lifting a 4 kg backpack from floor to Shelf 1 (1 m high) requires work W₁ = mgh₁ = 4×10×1 = 40 J against gravity, while lifting the same backpack to Shelf 2 (3 m high) requires work W₂ = mgh₂ = 4×10×3 = 120 J; comparing these: W₂/W₁ = 120/40 = 3, so the work to Shelf 2 is exactly 3 times the work to Shelf 1 because height is 3 times larger (work is directly proportional to height for same mass). Choice B is correct because it accurately states that work to Shelf 2 is 3 times work to Shelf 1 due to the 3× height difference, recognizing that work against gravity W = mgh is directly proportional to height. Choice A incorrectly claims work is the same (ignoring height difference), Choice C wrongly suggests less work for greater height (speed doesn't affect work calculation, only force and distance matter), and Choice D incorrectly states no work is done against gravity (gravity clearly does negative work as object rises, requiring positive work input). Work by non-contact forces demonstrates energy transfer without contact: gravitational force acts at distance throughout both lifts, and the work done (40 J vs 120 J) is stored as gravitational potential energy at each height. Understanding that forces can do work at distance is essential: the 3× height requires 3× work against the same gravitational force, demonstrating that work depends on distance moved against the force, not on contact between objects.

This question tests understanding that non-contact forces (gravity, electric, magnetic) can do work over distances, changing potential energy even when objects aren't touching. Work is defined as force applied over distance (W = F×d, or W = mgh for lifting against gravity), and non-contact forces like gravity, electric forces, and magnetic forces can do work because they act across space without requiring contact: when a ball falls, Earth's gravitational force (pulling downward at distance—ball not touching Earth) acts over the 5 m height it falls through, and gravity does work W = mgh on the ball, converting gravitational potential energy to kinetic energy. When the ball falls from rest down 5 m, gravitational force (pulling down) acts in the direction of motion (ball moving down), so gravity does positive work W = mgh (force and displacement in same direction: both downward); this positive work by gravity decreases gravitational PE (PE drops by mgh as height decreases) and increases kinetic energy (KE increases from 0 to mgh as ball speeds up during fall), demonstrating work-energy theorem: work done by gravity equals KE gained, and showing that PE is released as KE through gravity's work over the 5 m distance. Choice A is correct because it accurately explains that gravitational potential energy decreases (ball loses height, PE = mgh decreases) and kinetic energy increases (ball speeds up from rest, gaining KE) as gravity does positive work during the fall. Choice B reverses the energy changes (PE can't increase while falling), Choice C claims both energies increase (violates conservation—total mechanical energy stays constant), and Choice D incorrectly states no work is done because the ball isn't touching Earth (gravity clearly does work at a distance, as proven by the ball's acceleration and energy changes). Work by non-contact forces demonstrates energy transfer without contact: gravitational force does work as objects rise or fall (lift: work against gravity → PE increases, fall: work by gravity → PE decreases, KE increases), occurring across gaps without physical contact because gravity's field extends through space allowing force to act at distance. Understanding that forces can do work at distance is essential: as the ball falls 5 m, gravity continuously does work converting PE to KE, demonstrating that non-contact forces can transfer energy just as effectively as contact forces.

This question tests understanding that non-contact forces (gravity, electric, magnetic) can do work over distances, changing potential energy even when objects aren't touching. Work is defined as force applied over distance (W = F×d, or W = mgh for lifting against gravity), and non-contact forces like gravity, electric forces, and magnetic forces can do work because they act across space without requiring contact: when you lift an object from floor to height h, Earth's gravitational force (pulling downward at distance—object not touching Earth) acts over the height h you lift through, and you do work W = mgh against this force, storing the work as gravitational potential energy (PE = mgh gained). Lifting a 2 kg object from h=0 m to h=3 m at constant speed requires work against Earth's gravitational force: the change in gravitational potential energy is ΔPE = mgh_final - mgh_initial = (2×10×3) - (2×10×0) = 60 - 0 = +60 J; this positive change means PE increased by 60 J, which equals the work done against gravity during the lift (work against force stores as PE). Choice C is correct because it correctly calculates the change in gravitational potential energy as +60 J, recognizing that lifting increases PE (positive change) by the amount of work done against gravity. Choice A (+6 J) calculates incorrectly (appears to forget factor of g), Choice B (-60 J) has the wrong sign (PE increases when lifting, not decreases), and Choice D (0 J) incorrectly suggests no energy change despite the object clearly gaining height and PE. Work by non-contact forces demonstrates energy transfer without contact: gravitational force does work as objects rise or fall (lift: work against gravity → PE increases by mgh, fall: work by gravity → PE decreases by mgh), and the +60 J change represents energy stored in the gravitational field due to the object's higher position. Understanding that forces can do work at distance (not just contact forces) is essential for energy analysis: the 60 J of work done against the non-contact gravitational force is stored as gravitational potential energy, ready to be converted back to kinetic energy if the object falls.