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Concept · Mechanics
Newton'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
Switches you to Physics
Named traps
22 ways this goes wrong.
Each is a named misconception the questions are built to catch. Open one for the move that tempts, and the fix.
- 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.→
Key formulas
No formula sheet is matched to this concept yet. Work it from first principles, and watch the traps.
Shaky underneath?
If the concept underneath still feels loose, patch it here before coming back to CSCA questions.
FoundationsMechanics foundationsPhysics 2 breadthAdvanced mechanics support