Common Mistakes

Traps in Mechanics

6 mistake patterns students fall for. 3 high-frequency traps appear in almost every exam.

Centripetal Force as a Separate Force

Very CommonFORMULA

Adding 'centripetal force' as an extra force in the FBD alongside tension, gravity, and normal force.

Why: Students learn about centripetal force as a named concept and treat it like gravity or friction, but it is actually the net radial component of real forces.

WRONG: Draw FBD with T, mg, and F_centripetal as three separate forces

RIGHT: Draw only real forces (T, mg, N, f). The net radial component of these real forces provides the centripetal acceleration: net radial force = mv^2/r.

See pattern: Circular Motion Problems →

Using f = mu*N When Body is Static

Very CommonFORMULA

Using f = mu_s * N even when the body is not on the verge of sliding.

Why: Students memorize f = mu*N without noting that static friction adjusts from 0 to mu_s*N. It equals mu_s*N only at the verge of sliding.

WRONG: A 5 kg block on a surface (mu_s = 0.4) with 10 N applied force: f = 0.4 x 50 = 20 N

RIGHT: Check: max static friction = mu_s * N = 20 N. Applied force = 10 N < 20 N. So body doesn't move, and friction = 10 N (not 20 N).

See pattern: Friction Problems →

Applying Energy Conservation with Friction

Very CommonFORMULA

Using KE1 + PE1 = KE2 + PE2 when friction is present.

Why: Students apply the formula mechanically without checking if non-conservative forces do work.

WRONG: Block slides down rough incline: mgh = (1/2)mv^2

RIGHT: With friction: mgh - f*d = (1/2)mv^2, where f*d is the work done against friction. Or use work-energy theorem: W_net = delta KE.

See pattern: Work-Energy Theorem Application →

Wrong Coefficient of Restitution Direction

CommonFORMULA

Applying e along the wrong direction in oblique collisions.

Why: Students apply e to total velocities instead of components along the line of impact.

WRONG: In oblique collision, using total speeds to compute e

RIGHT: e applies ONLY along the line of impact (normal to surface at contact). Velocity components perpendicular to this line remain unchanged.

See pattern: Collision and Momentum →

Vertical Circular Motion: Rod vs String

CommonFORMULA

Using v_min = sqrt(gR) at the top for both string and rod.

Why: Students learn the string formula and apply it universally, not realizing that a rod can push (provide negative normal force).

WRONG: For a ball on a rod in vertical circle: v_min at top = sqrt(gR)

RIGHT: For string: T >= 0 at top, so v_min = sqrt(gR). For rod: rod can push, so v_min at top = 0. The required v at bottom changes accordingly.

See pattern: Circular Motion Problems →

Action-Reaction Pair on Same Body

CommonFORMULA

Treating weight and normal force as Newton's third law pair.

Why: Both have equal magnitude and opposite direction on a body at rest, making them look like an action-reaction pair.

WRONG: N = mg because they are action-reaction pairs (Newton's third law)

RIGHT: N = mg from Newton's second law (net force = 0 for equilibrium). The action-reaction pair of weight mg is the gravitational pull of the body on the Earth.

See pattern: FBD and Force Resolution →

Test yourself

Can you spot these traps under time pressure?

Take a timed quiz on Mechanics and see if you avoid the mistakes above.

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