Chapters

1 · Formulas and when each holds

• τ = r × F, with r from the point the torque is taken about.
• τ = Iα about a fixed axis, or about the centre of mass.
• L = Iω about a fixed axis; L is conserved when the net external torque is zero.
• Rolling without slipping: v = ωR, KE = ½mv²(1 + k²/R²).
• Parallel axis: I = I_cm + Md².

2 · Question types and the first step

• Rolling down an incline: write a = g sin θ / (1 + k²/R²) first.
• Collision with a rod: conserve angular momentum about the pivot, not linear momentum.

3 · Traps

Parallel axis: d is the distance between parallel axes, never along the axis.

Torque about a point: he took r from the axis instead of from the point asked about. Rule: mark the point first, draw r from it.

4 · My additions

• Draw the axis before writing any I.