UPCAT Physics — Newton's Laws, Dynamics & MomentumMisconception Buster
Avoid the most common Newton's Laws, Dynamics & Momentum mistakes made by UPCAT reviewers. Each misconception here has been pulled from real UPCAT Physics questions where University of the Philippines used it to separate strong reviewers from weak ones. Learn these before your next mock.
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 Newton's Laws, Dynamics & Momentum appears in position 3rd 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.
Newton's Laws, Dynamics & Momentum - Misconception buster
Newton's Laws and momentum concepts are among the most misunderstood topics in physics, leading to massive point losses in UPCAT and other entrance exams. Students often rely on 'common sense' which directly contradicts physical reality. This guide exposes the most dangerous misconceptions that trick even smart students into wrong answers, showing you exactly where your intuition will betray you and how to think like a physicist instead.
Summary
The biggest physics misconceptions stem from trusting everyday intuition over scientific reasoning. Heavy objects don't fall faster - that's air resistance, not gravity. Forces don't create motion - they create acceleration. Third Law pairs act on different objects so they can't cancel. Weight and mass are completely different quantities. Master these conceptual traps and your physics scores will soar above students who rely on 'common sense' instead of Newton's precise laws.
Misconceptions
Heavier objects fall faster than lighter objects
Tags
- common_error
- conceptual_gap
- gravity_misconception
Topic
Newton's Second Law and Gravity
Severity
critical
Exam Impact
Students choose wrong answers in free-fall problems, incorrectly calculate acceleration for different masses, and fail to recognize when air resistance can be ignored
The Reality
All objects fall at the same rate in a vacuum (9.8 m/s²). The rock-feather difference is due to air resistance, not weight. Gravity does pull harder on heavier objects (F = mg), but heavier objects also have more inertia (resistance to acceleration). These effects exactly cancel out: a = F/m = mg/m = g
Trap Question
Question
A 5kg bowling ball and a 0.5kg tennis ball are dropped simultaneously from a 10m height in a vacuum. Which hits the ground first?
Explanation
In a vacuum, all objects fall with the same acceleration g = 9.8 m/s², regardless of their mass. Both balls will take t = √(2h/g) = √(20/9.8) = 1.43 seconds to fall
Wrong Answer
The bowling ball hits first because it's heavier
Correct Answer
Both hit the ground at exactly the same time
Misconception Id
M1
Correct Vs Incorrect
Correct Approach
Both balls experience a = g = 9.8 m/s² regardless of mass. The 10kg ball has 10x more force (98N vs 9.8N) but also 10x more mass to accelerate, so acceleration is identical
Incorrect Approach
A 10kg ball and 1kg ball are dropped. Student thinks: 'Heavier ball has 10x more gravitational force, so it accelerates 10x faster'
Why Students Believe It
Daily experience shows that a rock falls faster than a feather, and our intuition tells us that heavier things should fall faster because gravity 'pulls harder' on them
Force is needed to keep an object moving at constant velocity
Tags
- newton_first_law
- conceptual_gap
- force_misconception
Topic
Newton's First Law
Severity
critical
Exam Impact
Students incorrectly identify forces in free-body diagrams, think net force exists when velocity is constant, and misunderstand equilibrium conditions
The Reality
Newton's First Law states that objects in motion stay in motion unless acted upon by an unbalanced force. In real life, friction opposes motion, so we need applied force to balance friction, not to maintain motion. In space (no friction), one push sets an object moving forever
Trap Question
Question
A box slides across a frictionless surface at constant 5 m/s. What is the net force on the box?
Explanation
Constant velocity means zero acceleration (a = 0). By Newton's Second Law, F_net = ma = m(0) = 0. No force is needed to maintain constant velocity - this is Newton's First Law
Wrong Answer
There must be a 5N force to maintain the 5 m/s motion
Correct Answer
The net force is 0N
Misconception Id
M2
Correct Vs Incorrect
Correct Approach
Engine force balances friction/air resistance. Net force = 0, so velocity stays constant. If friction disappeared, car would coast at 60 km/h forever with no engine force
Incorrect Approach
Car moving at 60 km/h needs engine force to maintain speed. Student thinks this proves 'force creates constant velocity'
Why Students Believe It
In everyday life, we must keep pushing a cart or pedaling a bike to maintain constant speed. When we stop applying force, things slow down and stop
Mass and weight are the same thing
Tags
- unit_confusion
- mass_vs_weight
- common_error
Topic
Newton's Second Law and Gravity
Severity
major
Exam Impact
Students use wrong units in calculations, confuse force and mass in Newton's Second Law problems, and make errors in momentum and energy calculations
The Reality
Mass is the amount of matter in an object (measured in kg), constant everywhere. Weight is the gravitational force on that mass (measured in Newtons), varies with location. W = mg. Your mass is the same on Earth and Moon, but your weight is 6x less on the Moon
Trap Question
Question
An astronaut has a mass of 80kg on Earth. What is their weight on the Moon where g = 1.6 m/s²?
Explanation
Weight = mg = 80kg × 1.6 m/s² = 128N. Mass remains 80kg, but weight changes because gravitational acceleration is different. Weight is a force (Newtons), not a mass (kg)
Wrong Answer
80kg (because mass and weight are the same)
Correct Answer
128N
Misconception Id
M3
Correct Vs Incorrect
Correct Approach
70kg is mass (constant). Weight on Earth = mg = 70 × 9.8 = 686N. On Moon: same mass (70kg) but weight = 70 × 1.6 = 112N
Incorrect Approach
A 70kg person thinks their weight is 70kg everywhere. Uses F = ma as F = 70a instead of F = (70 × 9.8)a
Why Students Believe It
In everyday language, we use 'mass' and 'weight' interchangeably. Scales show kilograms, making students think mass and weight have the same units
Newton's Third Law means equal and opposite forces cancel out
Tags
- newton_third_law
- force_pairs
- conceptual_gap
Topic
Newton's Third Law
Severity
major
Exam Impact
Students incorrectly conclude no motion is possible, misidentify action-reaction pairs in free-body diagrams, and fail to properly analyze collision problems
The Reality
Newton's Third Law forces act on DIFFERENT objects, so they don't cancel. When you push a wall, you push the wall and the wall pushes you back - these are on different objects. Motion happens when forces on the SAME object don't balance
Trap Question
Question
You push a 5kg box with 20N force. The box pushes back on you with 20N. Why does the box accelerate?
Explanation
Forces only cancel when they act on the same object. The 20N on the box (minus friction) gives it acceleration a = F_net/m. The 20N on you is balanced by your friction with the ground
Wrong Answer
The forces are equal and opposite, so they cancel out and the box shouldn't move
Correct Answer
The 20N forces act on different objects (you and the box), so they don't cancel
Misconception Id
M4
Correct Vs Incorrect
Correct Approach
I push box with 10N (force on box). Box pushes me with 10N (force on me). These act on different objects. Box moves if my 10N push exceeds friction on the box
Incorrect Approach
Student thinks: 'I push box with 10N, box pushes back with 10N, forces cancel, box can't move'
Why Students Believe It
Students hear 'equal and opposite forces' and think these forces act on the same object, canceling each other out, resulting in no motion
Momentum is always conserved in collisions
Tags
- momentum_conservation
- external_forces
- collision_analysis
Topic
Conservation of Momentum
Severity
major
Exam Impact
Students apply conservation incorrectly when friction is present, ignore external forces in collision problems, and get wrong answers in real-world scenarios
The Reality
Momentum is only conserved when the net external force on the system is zero. If external forces like friction, gravity, or normal forces act during collision, momentum is not conserved. Always check for external forces first
Trap Question
Question
A 1000kg car traveling at 10 m/s collides with a stationary 1500kg truck on a road with friction. After collision, they stick together. Can you use conservation of momentum to find their final velocity?
Explanation
Conservation of momentum requires no external forces. Road friction acts on the car-truck system during collision, violating this condition. You need to account for the friction impulse
Wrong Answer
Yes, p_initial = p_final: (1000)(10) + 0 = (2500)v_f, so v_f = 4 m/s
Correct Answer
No, because friction with the road is an external force that changes the system's momentum
Misconception Id
M5
Correct Vs Incorrect
Correct Approach
Check external forces first. Road friction acts on both cars during collision, so momentum is NOT conserved. Must use impulse-momentum theorem including friction
Incorrect Approach
Two cars collide on a road. Student automatically applies p_initial = p_final, ignoring road friction during collision
Why Students Believe It
Physics textbooks emphasize conservation of momentum, so students think it applies to every collision situation without considering external forces
A larger force always produces larger acceleration
Tags
- formula_misuse
- acceleration_calculation
- mass_effect
Topic
Newton's Second Law
Severity
major
Exam Impact
Students make calculation errors in F = ma problems, incorrectly compare accelerations of different objects, and miss the inverse relationship between mass and acceleration
The Reality
Acceleration depends on BOTH force AND mass: a = F/m. A small force on a small mass can produce larger acceleration than a large force on a large mass. The ratio F/m determines acceleration, not just F
Trap Question
Question
A 50N force acts on a 10kg object. A 20N force acts on a 2kg object. Which object has greater acceleration?
Explanation
Object 1: a = F/m = 50N/10kg = 5 m/s². Object 2: a = 20N/2kg = 10 m/s². The smaller object accelerates twice as much despite experiencing less force
Wrong Answer
The 10kg object because 50N > 20N
Correct Answer
The 2kg object has greater acceleration
Misconception Id
M6
Correct Vs Incorrect
Correct Approach
Elephant (1000kg): a = 100N/1000kg = 0.1 m/s². Mouse (0.1kg): a = 10N/0.1kg = 100 m/s². Mouse accelerates 1000x more despite smaller force
Incorrect Approach
100N force on elephant vs 10N force on mouse. Student thinks elephant accelerates more because force is larger
Why Students Believe It
F = ma seems to directly state that force and acceleration are proportional, so students ignore the role of mass
Velocity and acceleration are the same thing
Tags
- velocity_vs_acceleration
- conceptual_gap
- motion_analysis
Topic
Kinematics and Motion
Severity
major
Exam Impact
Students confuse v and a in kinematics equations, misinterpret motion graphs, and make errors in force calculations
The Reality
Velocity is how fast position changes (m/s). Acceleration is how fast velocity changes (m/s²). An object can have high velocity but zero acceleration (constant speed), or zero velocity but high acceleration (starting from rest)
Trap Question
Question
A car travels at constant 30 m/s for 10 seconds. What is its acceleration?
Explanation
Constant velocity means no change in velocity, so acceleration = Δv/Δt = 0/10 = 0 m/s². High velocity doesn't mean high acceleration
Wrong Answer
30 m/s² (because velocity equals acceleration)
Correct Answer
0 m/s²
Misconception Id
M7
Correct Vs Incorrect
Correct Approach
Constant velocity means acceleration = 0. Car at 60 km/h with a = 0 vs car starting from rest with a = 5 m/s² - very different situations
Incorrect Approach
Car traveling at constant 60 km/h. Student thinks 'high velocity means high acceleration'
Why Students Believe It
Both involve motion and have similar units (m/s vs m/s²), and students often use the terms interchangeably in everyday language
Objects at rest have no forces acting on them
Tags
- static_equilibrium
- force_balance
- free_body_diagrams
Topic
Force Analysis and Equilibrium
Severity
major
Exam Impact
Students miss forces in free-body diagrams, incorrectly analyze equilibrium problems, and fail to identify all forces in static situations
The Reality
Objects at rest often have multiple forces acting on them that are balanced (net force = 0). A book on a table has gravity pulling down and normal force pushing up. Newton's First Law applies: net force = 0 means constant velocity (including zero velocity)
Trap Question
Question
A 2kg book rests on a table. What is the net force on the book?
Explanation
Weight = mg = 2×9.8 = 19.6N downward. Normal force = 19.6N upward. Net force = 19.6N - 19.6N = 0N. Multiple forces exist but they balance
Wrong Answer
There are no forces because the book isn't moving
Correct Answer
The net force is 0N, but individual forces exist
Misconception Id
M8
Correct Vs Incorrect
Correct Approach
Book has weight (mg) downward and normal force (N) upward. N = mg, so net force = 0, explaining why book stays at rest
Incorrect Approach
Book sitting on table. Student draws no forces because 'it's not moving so no forces act'
Why Students Believe It
If an object isn't moving, students assume no forces are present because 'forces cause motion'
Impulse only occurs during collisions
Tags
- impulse_applications
- momentum_change
- force_time
Topic
Impulse and Momentum
Severity
minor
Exam Impact
Students miss impulse applications in non-collision problems and fail to connect impulse-momentum theorem to everyday situations
The Reality
Impulse (J = FΔt) occurs whenever a force acts over time, whether gradual or sudden. Pushing a cart, throwing a ball, or braking a car all involve impulse. Impulse equals change in momentum for any situation
Trap Question
Question
A 0.5kg ball is thrown upward, reaching maximum height in 2 seconds. What impulse did the thrower apply?
Explanation
At max height, v = 0, so initial throw velocity was v = gt = 9.8×2 = 19.6 m/s upward. Impulse = change in momentum from 0 to mv
Wrong Answer
No impulse because this isn't a collision
Correct Answer
Impulse = Δp = m(v_final - v_initial) = 0.5(19.6 - 0) = 9.8 N⋅s
Misconception Id
M9
Correct Vs Incorrect
Correct Approach
Recognize impulse in: athlete throwing javelin (muscle force over time), car braking (friction force over time), rocket launching (thrust force over time)
Incorrect Approach
Student only thinks about impulse when objects collide or impact each other
Why Students Believe It
Textbooks often introduce impulse in the collision chapter, making students think impulse is only relevant for sudden impacts
Friction always opposes motion
Tags
- friction_direction
- static_friction
- motion_mechanics
Topic
Friction Forces
Severity
minor
Exam Impact
Students incorrectly identify friction direction in complex situations, misunderstand walking/rolling mechanics, and make errors in inclined plane problems
The Reality
Friction opposes RELATIVE motion or attempted relative motion. Static friction can actually cause motion (like friction between your shoes and ground when you walk) or prevent motion (keeping objects from sliding down inclines)
Trap Question
Question
When you walk forward, in which direction does friction act on your feet?
Explanation
Your feet try to slide backward relative to the ground. Static friction opposes this relative motion by acting forward on your feet, providing the force that pushes you forward
Wrong Answer
Backward, because friction always opposes motion
Correct Answer
Forward, enabling you to walk
Misconception Id
M10
Correct Vs Incorrect
Correct Approach
Feet try to slide backward relative to ground, so friction acts forward on feet, enabling walking. Friction provides the force that accelerates you forward
Incorrect Approach
Person walking forward. Student thinks friction acts backward on feet, opposing motion
Why Students Believe It
Most examples show friction slowing things down, like sliding blocks or rolling balls coming to rest
Quick Self Check
Without air resistance, all objects fall with the same acceleration g = 9.8 m/s², regardless of mass
Statement
In a vacuum, a hammer and feather dropped from the same height will hit the ground at the same time
Constant velocity means zero acceleration, which requires zero net force by Newton's First Law
Statement
A car traveling at constant speed on a highway has zero net force acting on it
Mass is the amount of matter and remains constant. Only weight (gravitational force) changes
Statement
Your mass changes when you travel from Earth to the Moon
Third Law forces act on different objects, so they cannot cancel each other
Statement
Newton's Third Law forces always cancel each other out
Momentum is only conserved when no external forces act on the system
Statement
Momentum is conserved in all collision situations
Objects at rest often have balanced forces (like weight and normal force) that sum to zero
Statement
An object at rest has no forces acting on it
Acceleration can be opposite to velocity (like when braking) or perpendicular (like circular motion)
Statement
Acceleration is always in the same direction as velocity
Friction acts forward on your feet, providing the force that enables you to walk forward
Statement
Friction between your shoes and the ground acts backward when you walk forward
Ready to practise for the UPCAT 2026?
Super Tutor's AI review plan adapts to your weak areas and builds a weekly practice schedule around your target UPCAT exam date.