ConceptMechanics
← All conceptsNewton's laws, forces & equilibrium
free-body diagrams, weight/normal/tension/friction (static vs kinetic, f=μ·N), inclined-plane resolution, parallelogram/orthogonal composition, action-reaction vs balanced-force distinction, equilibrium of concurrent forces
22 ways this goes wrong. Each one is a named misconception the questions are built to catch.
- Action-reaction on same bodyPairs two forces acting on the same object as an action-reaction pair, so they wrongly cancel.→
- Action-reaction pair cancelsConcludes nothing can accelerate because action and reaction are equal and opposite, ignoring that they act on different bodies.→
- Apparent weight ignores accelerationEquates a lift scale reading to mg regardless of the lift's upward or downward acceleration.→
- Centrifugal force as real outward forceAdds an outward 'centrifugal' force to a free-body diagram in an inertial frame.→
- Component sin/cos swappedResolves a force using sin where cos is needed (or vice versa) for the given angle reference.→
- Force omitted from FBDLeaves a real force (e.g. tension, friction, or normal) out of the free-body diagram when summing forces.→
- Forces added as scalarsAdds force magnitudes arithmetically instead of as vectors when composing or checking equilibrium.→
- Friction depends on contact areaBelieves friction force increases with contact surface area rather than depending on normal force and μ.→
- Friction opposes the push not motionDirects friction opposite the applied force rather than opposite the relative motion or its tendency.→
- Heavier exerts larger forceThinks the more massive (or faster) body in a contact/collision exerts a larger force on the other than it receives.→
- Incline sin/cos swappedUses mg cos(θ) along the incline and mg sin(θ) perpendicular, swapping the weight components.→
- Inertia depends on speedBelieves a faster-moving object has more inertia, confusing inertia (mass) with momentum.→
- Kinetic/static μ swappedUses the static coefficient for a sliding body or the kinetic one for a body on the verge of moving.→
- Mass vs weight confusedUses mass where weight (mg) is needed, or quotes weight in kilograms, conflating the two quantities.→
- Motion requires a net forceAssumes a body moving at constant velocity must have a forward net force, violating Newton's first law.→
- N=mg on an inclineSets the normal force to mg on a slope instead of mg cos(θ).→
- Net force along motionBelieves the net force must point in the direction of motion rather than in the direction of acceleration.→
- Normal force always equals weightSets the normal force equal to weight even on an incline or when an extra vertical force acts.→
- Only one axis balancedBalances forces along one axis only, forgetting that both x and y component sums must be zero for equilibrium.→
- Static friction always at maximumUses μs·N as the static friction value even when the applied force is below the maximum (does not check fstatic ≤ fmax).→
- Tension varies in ideal stringAssumes tension changes along a single massless string over a frictionless pulley.→
- Weight and normal mis-pairedCalls an object's weight and the normal force on it an action-reaction pair, though both act on the same body.→
