Trap 1: Sphere Tangent

Q: Moment of inertia of a solid sphere about a tangent to the sphere.

Trap answer: $2MR^2/5$ (students give the diameter value) Correct answer: $7MR^2/5$ (apply parallel axis theorem: $2MR^2/5 + MR^2$)

Why students fall for it: They recall $I_{sphere} = 2MR^2/5$ but fail to apply the parallel axis theorem for the shifted axis.

Trap 2: All Rolling Bodies Reach at the Same Time

Q: Solid sphere, disc, and ring roll down simultaneously. Order?

Trap answer: All at the same time (students think mass cancels so everything is equal) Correct answer: Solid sphere first, then disc, then ring

Why: Mass cancels, but the shape factor $K^2/R^2$ does NOT cancel.

Trap 3: Angular Momentum Conservation Implies KE Conservation

Q: Skater pulls arms in. Is KE conserved?

Trap answer: Yes (students confuse L conservation with energy conservation) Correct answer: No. KE increases. $KE = L^2/(2I)$; as $I$ decreases (L = const), KE increases.

Trap 4: Perpendicular Axis Theorem for a Sphere

Q: Can we apply $I_z = I_x + I_y$ to a solid sphere?

Trap answer: Yes (students think the theorem is universal) Correct answer: No. Only for flat planar bodies.

Trap 5: Ring is Fastest Because It Has the Most Mass at the Edge

Q: Rolling race: which is fastest?

Trap answer: Ring (students think maximum concentration at R = most force) Correct answer: Solid sphere (smallest $K^2/R^2$ = largest acceleration).