UPCAT Physics — Work, Energy & ImpulseMisconception Buster
If you have been missing Work, Energy & Impulse questions on your UPCAT mocks, the cause is almost always a misconception. This page lists the ones University of the Philippines exploits most often in the UPCAT Physics subtest and shows how to correct them before exam day.
Exam context
The University of the Philippines College Admission Test is conducted by University of the Philippines and is scheduled for Mid-2026 (announced by UP Admissions). The Physics subtest is marked as "Core" in the official pattern, and Work, Energy & Impulse appears in position 4th of 6 in the UPCAT Physics review rotation. Passing mark: UPG ≤ 2.2 typical. Recent UPCAT 2026 papers have drawn roughly 20 questions from this subject.
Work, Energy & Impulse - Misconception buster
Work, Energy & Impulse is a high-scoring topic in UPCAT Physics, but students often lose marks due to fundamental misconceptions about these concepts. Many believe they understand these topics because they seem intuitive, but exam questions reveal deep conceptual gaps. Understanding these misconceptions is crucial because they appear in 15-20% of UPCAT Physics questions and can cost you valuable points that determine your college admission.
Summary
The biggest mistake students make with Work, Energy & Impulse is treating these concepts as simple memorized formulas instead of understanding their physical meaning. Remember: work depends on the angle between force and displacement, kinetic energy increases with the square of velocity, mechanical energy is conserved only without friction, impulse is the change in momentum (not momentum itself), and efficiency can never exceed 100%. Always check if your answers make physical sense - if efficiency is over 100% or if you're ignoring friction in a realistic problem, you've made an error. Master these misconceptions and you'll avoid the traps that catch most students in UPCAT Physics.
Misconceptions
Work is always force times distance, regardless of direction
Tags
- formula_confusion
- vector_concepts
- common_error
Topic
Work
Severity
critical
Exam Impact
Students incorrectly calculate work in scenarios where force and displacement are not parallel, leading to wrong answers in 30-40% of work-related problems.
The Reality
Work is W = F·d·cos(θ) where θ is the angle between force and displacement. If force is perpendicular to motion (θ = 90°), no work is done because cos(90°) = 0. A person carrying a heavy bag while walking horizontally does zero work on the bag because the force is upward but displacement is horizontal.
Trap Question
Question
A student carries a 20kg backpack horizontally for 100m at constant velocity. How much work does the student do on the backpack? (g = 10 m/s²)
Explanation
The force applied by the student is upward (to support the weight) while displacement is horizontal. Since force and displacement are perpendicular, cos(90°) = 0, so no work is done on the backpack.
Wrong Answer
20,000 J (students calculate W = mgh = 20 × 10 × 100)
Correct Answer
0 J
Misconception Id
M1
Correct Vs Incorrect
Correct Approach
A 50N force pushes a box 10m at 60° angle. Work = 50 × 10 × cos(60°) = 50 × 10 × 0.5 = 250J
Incorrect Approach
A 50N force pushes a box 10m at 60° angle. Work = 50 × 10 = 500J
Why Students Believe It
Students memorize W = Fd without understanding that work requires force and displacement in the same direction. They think any force applied over any distance equals work.
Kinetic energy increases linearly with velocity
Tags
- quadratic_relationship
- common_error
- calculation_mistake
Topic
Kinetic Energy
Severity
critical
Exam Impact
Students make major calculation errors in collision problems, energy conservation, and stopping distance problems, affecting 25-30% of energy-related questions.
The Reality
Kinetic energy increases with the square of velocity: KE = ½mv². Doubling velocity quadruples kinetic energy. If a car at 20 m/s has 400,000 J of KE, the same car at 40 m/s has 1,600,000 J of KE.
Trap Question
Question
A 1000kg car traveling at 20 m/s has kinetic energy of 200,000 J. What is its kinetic energy when traveling at 40 m/s?
Explanation
KE = ½mv². When velocity doubles, v² becomes 4 times larger, so KE becomes 4 times larger: 200,000 × 4 = 800,000 J.
Wrong Answer
400,000 J (students think KE doubles when velocity doubles)
Correct Answer
800,000 J
Misconception Id
M2
Correct Vs Incorrect
Correct Approach
If speed doubles from 10 m/s to 20 m/s, KE quadruples from 500J to 2000J because KE ∝ v²
Incorrect Approach
If speed doubles from 10 m/s to 20 m/s, KE doubles from 500J to 1000J
Why Students Believe It
Students assume that doubling speed doubles kinetic energy, similar to how doubling distance doubles displacement. This linear thinking feels natural.
Potential energy depends on the path taken to reach a height
Tags
- path_independence
- conceptual_gap
- energy_conservation
Topic
Potential Energy
Severity
major
Exam Impact
Students incorrectly calculate energy changes in problems involving different paths, affecting energy conservation problems and efficiency calculations.
The Reality
Gravitational potential energy PE = mgh depends only on mass, gravitational acceleration, and height above reference point. The path taken is irrelevant. Whether you walk up a ramp, climb stairs, or take an elevator to reach 10m height, your PE increase is the same.
Trap Question
Question
Two identical 70kg hikers reach a 500m mountain peak. Hiker A takes a steep direct path (2km), Hiker B takes a winding gentle path (8km). Compare their potential energies at the top.
Explanation
Potential energy depends only on height above reference point, not on the path taken to reach that height. Both hikers are at the same height, so they have identical potential energy.
Wrong Answer
Hiker B has higher PE because they traveled a longer distance
Correct Answer
Both hikers have the same PE = mgh = 70 × 10 × 500 = 350,000 J
Misconception Id
M3
Correct Vs Incorrect
Correct Approach
Both persons reach the same height of 10m, so both have the same PE = mgh regardless of path taken
Incorrect Approach
Person A climbs 100 steps (longer path) to reach 10m height, Person B takes elevator (shorter path). PE_A > PE_B
Why Students Believe It
Students confuse work (which can depend on path) with potential energy. They think climbing stairs versus taking an elevator to the same height gives different potential energies.
Impulse and momentum are the same thing
Tags
- concept_confusion
- definition_error
- sign_convention
Topic
Impulse and Momentum
Severity
major
Exam Impact
Students confuse formulas and concepts in collision problems, incorrectly applying impulse formulas when momentum formulas are needed, affecting 20-25% of impulse-momentum problems.
The Reality
Impulse (J = FΔt) is the change in momentum, not momentum itself. Momentum p = mv is the quantity of motion an object possesses. Impulse is what causes momentum to change. A moving object has momentum; a force applied over time creates impulse.
Trap Question
Question
A 0.5kg baseball moving at 30 m/s is caught by a glove in 0.1s. What was the impulse applied by the glove?
Explanation
Impulse = change in momentum = final momentum - initial momentum = 0 - (0.5 × 30) = -15 kg·m/s. The negative sign indicates the impulse opposes the initial motion.
Wrong Answer
15 kg·m/s (students give the initial momentum instead of impulse)
Correct Answer
-15 kg·m/s
Misconception Id
M4
Correct Vs Incorrect
Correct Approach
A 2kg ball moving at 5 m/s has momentum = 10 kg·m/s. Impulse would be the change in this momentum due to applied force.
Incorrect Approach
A 2kg ball moving at 5 m/s has impulse = 10 kg·m/s
Why Students Believe It
Students see J = Δp and think impulse equals momentum change, so they use the terms interchangeably. Both have similar units (kg·m/s) which adds confusion.
Energy is always conserved in all situations
Tags
- overgeneralization
- non_conservative_forces
- friction_effects
Topic
Energy Conservation
Severity
critical
Exam Impact
Students incorrectly apply conservation of mechanical energy in problems with friction or air resistance, leading to wrong answers in 35-40% of energy conservation problems.
The Reality
Total energy is always conserved, but mechanical energy (KE + PE) is conserved only when no non-conservative forces (friction, air resistance, applied forces) do work. When friction is present, mechanical energy converts to heat energy.
Trap Question
Question
A 2kg block slides down a 5m high rough incline and reaches the bottom with speed 6 m/s. Is mechanical energy conserved? (g = 10 m/s²)
Explanation
Initial PE = mgh = 2×10×5 = 100J. Final KE = ½mv² = ½×2×36 = 36J. Since final KE < initial PE, 64J was lost to friction as heat. Mechanical energy decreased.
Wrong Answer
Yes, because energy cannot be destroyed
Correct Answer
No, mechanical energy is not conserved
Misconception Id
M5
Correct Vs Incorrect
Correct Approach
Ball slides down rough incline: Initial PE = Final KE + Energy lost to friction
Incorrect Approach
Ball slides down rough incline: Initial PE = Final KE (ignoring friction)
Why Students Believe It
Students learn 'energy cannot be created or destroyed' and overapply this principle, thinking mechanical energy is conserved even when friction, air resistance, or other non-conservative forces are present.
Power is the same as energy
Tags
- unit_confusion
- rate_vs_quantity
- calculation_error
Topic
Power
Severity
major
Exam Impact
Students make unit errors and calculation mistakes in power-related problems, incorrectly converting between energy and power in 20-25% of related questions.
The Reality
Power is the rate of energy transfer or work done: P = W/t or P = E/t. Energy is the capacity to do work, while power is how quickly that energy is used or work is done. A 100W light bulb uses 100J of energy every second.
Trap Question
Question
A 500W electric motor runs for 10 minutes. How much energy does it consume?
Explanation
Energy = Power × time = 500W × 600s = 300,000 J. Power tells us the rate of energy consumption, not the total energy used.
Wrong Answer
500 J (students think power equals energy)
Correct Answer
300,000 J
Misconception Id
M6
Correct Vs Incorrect
Correct Approach
A 1000W motor uses 1000J of energy every second, or does 1000J of work every second
Incorrect Approach
A 1000W motor has 1000J of energy
Why Students Believe It
Students confuse power and energy because both relate to 'strength' or 'capability' in everyday language. They use watts and joules interchangeably.
In collisions, kinetic energy is always conserved
Tags
- collision_types
- energy_loss
- conservation_laws
Topic
Collisions
Severity
critical
Exam Impact
Students incorrectly apply kinetic energy conservation in inelastic collision problems, leading to wrong answers in 30-35% of collision questions.
The Reality
Momentum is always conserved in collisions (if no external forces), but kinetic energy is conserved only in perfectly elastic collisions. In inelastic collisions, some kinetic energy converts to heat, sound, and deformation energy.
Trap Question
Question
Two identical 1kg balls collide head-on. Ball A (4 m/s) hits stationary Ball B, and they stick together. What is their final kinetic energy?
Explanation
This is a perfectly inelastic collision (they stick). Momentum: 1×4 + 1×0 = 2×v_final, so v_final = 2 m/s. Final KE = ½×(2kg)×(2m/s)² = 4J. Half the initial KE (8J) was lost.
Wrong Answer
8 J (assuming KE conservation: Initial KE = ½×1×16 = 8J)
Correct Answer
4 J
Misconception Id
M7
Correct Vs Incorrect
Correct Approach
In elastic collisions: Both KE and momentum conserved. In inelastic collisions: Only momentum conserved, KE decreases
Incorrect Approach
In all collisions: Initial KE = Final KE and Initial momentum = Final momentum
Why Students Believe It
Students learn that momentum is conserved in collisions and assume kinetic energy is also always conserved. They think all collisions are elastic.
Work-energy theorem only applies when net work is positive
Tags
- sign_convention
- theorem_application
- negative_work
Topic
Work-Energy Theorem
Severity
major
Exam Impact
Students incorrectly apply or avoid using work-energy theorem in problems involving friction, braking, or deceleration, affecting 25% of work-energy problems.
The Reality
The work-energy theorem W_net = ΔKE applies always, regardless of whether work is positive, negative, or zero. Negative net work means kinetic energy decreases (object slows down). Zero net work means kinetic energy stays constant.
Trap Question
Question
A 1000kg car traveling at 20 m/s comes to rest due to friction over 100m. What was the work done by friction?
Explanation
Using W_net = ΔKE: W_friction = KE_final - KE_initial = 0 - ½(1000)(20²) = -200,000 J. Negative work by friction removes kinetic energy.
Wrong Answer
Cannot calculate because work-energy theorem doesn't apply to negative work
Correct Answer
-200,000 J
Misconception Id
M8
Correct Vs Incorrect
Correct Approach
Car braking: W_net = ΔKE. Friction work is negative, so ΔKE is negative (speed decreases)
Incorrect Approach
Car braking: Cannot use work-energy theorem because work is negative
Why Students Believe It
Students think the work-energy theorem W_net = ΔKE only works when objects speed up (positive work). They get confused when objects slow down or work is negative.
Heavier objects always have more momentum
Tags
- mass_vs_velocity
- momentum_comparison
- conceptual_error
Topic
Momentum
Severity
major
Exam Impact
Students make incorrect predictions about collision outcomes and momentum comparisons in 20% of momentum-related problems.
The Reality
Momentum depends on both mass and velocity: p = mv. A light object moving very fast can have more momentum than a heavy object moving slowly. A 0.1kg bullet at 500 m/s has more momentum than a 10kg bowling ball at 2 m/s.
Trap Question
Question
Which has greater momentum: a 2000kg truck moving at 1 m/s or a 1kg ball moving at 3000 m/s?
Explanation
Truck momentum = 2000×1 = 2000 kg⋅m/s. Ball momentum = 1×3000 = 3000 kg⋅m/s. Despite being much lighter, the ball's high velocity gives it greater momentum.
Wrong Answer
The truck because it's much heavier
Correct Answer
The ball
Misconception Id
M9
Correct Vs Incorrect
Correct Approach
Compare p = mv for each object. 1kg at 50 m/s has more momentum than 10kg at 2 m/s
Incorrect Approach
10kg object always has more momentum than 1kg object
Why Students Believe It
Students focus only on the mass term in p = mv and think that larger mass automatically means larger momentum, ignoring the velocity component.
Efficiency can exceed 100%
Tags
- calculation_check
- conservation_violation
- percentage_error
Topic
Efficiency
Severity
minor
Exam Impact
Students make calculation errors or fail to check reasonableness of their efficiency answers in 15% of efficiency problems.
The Reality
Efficiency = (useful energy output/total energy input) × 100% can never exceed 100% because output cannot be greater than input. This would violate conservation of energy. Maximum theoretical efficiency is 100% (perfect machine with no energy losses).
Trap Question
Question
A motor uses 800J of electrical energy to lift a 10kg mass 5m high. Calculate the efficiency. (g = 10 m/s²)
Explanation
Useful work output = mgh = 10×10×5 = 500J. Efficiency = (500J/800J)×100% = 62.5%. Always check that efficiency is less than 100%.
Wrong Answer
If someone calculates backwards: 800J/500J = 160% efficiency
Correct Answer
62.5%
Misconception Id
M10
Correct Vs Incorrect
Correct Approach
This is impossible. Check calculations - either work output or energy input is wrong.
Incorrect Approach
Machine does 1200J work with 1000J input energy. Efficiency = 120%
Why Students Believe It
Students sometimes calculate efficiency incorrectly and get values over 100%, then think this is possible because they misunderstand what efficiency means.
Quick Self Check
Work requires force and displacement in the same direction. The force is vertical (upward) but displacement is horizontal, so no work is done on the box.
Statement
A person carrying a heavy box while walking horizontally at constant speed does work on the box
KE = ½mv². If velocity triples, v² becomes 9 times larger, so KE increases by factor of 9.
Statement
If a car's speed triples, its kinetic energy increases by a factor of 9
While both have units kg⋅m/s, impulse is the change in momentum (J = Δp), not momentum itself. Impulse causes momentum change.
Statement
Impulse and momentum have the same units and are essentially the same concept
Mechanical energy is conserved only when no non-conservative forces like friction do work. Friction converts mechanical energy to heat.
Statement
Mechanical energy is always conserved regardless of friction
Momentum is always conserved in collisions, but kinetic energy is conserved only in perfectly elastic collisions. In inelastic collisions, some KE is lost.
Statement
In all types of collisions, both momentum and kinetic energy are conserved
Power is measured in watts (J/s) while energy is measured in joules. Power is the rate of energy transfer.
Statement
Power and energy are measured in the same units
Momentum = mass × velocity. A light bullet with very high velocity can have greater momentum than a heavy ball with low velocity.
Statement
A light bullet can have more momentum than a heavy bowling ball
Efficiency cannot exceed 100% because this would violate conservation of energy. Output cannot be greater than input.
Statement
The efficiency of any real machine can theoretically reach 120% with good design
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