Physics - Mechanics
Physics - Mechanics tests whether you can translate a physical situation into equations and keep signs and units consistent. Expect kinematics graphs, Newton’s laws with friction or inclines, momentum conservation, work–energy, and uniform circular motion.
Core rules
- For constant acceleration: v = u + at, s = ut + ½at², v² = u² + 2as. Choose one positive direction and keep it.
- Newton’s second law: ΣF = ma; friction f ≤ μN, with equality at slipping.
- Momentum is conserved when external impulse is negligible; energy is conserved only for conservative forces.
- Circular motion needs a net inward force: a = v²/r.
Worked example
A ball is launched at 20 m/s at 30° above horizontal from level ground. Ignore air resistance and take g = 10 m/s². Resolve: u_x = 20 cos30° = 17.3 m/s; u_y = 20 sin30° = 10 m/s. At maximum height v_y = 0, so 0 = 10² − 2(10)h gives h = 5 m. Time to top: 0 = 10 − 10t, t = 1 s; total flight 2 s. Range = u_x × total time = 17.3 × 2 = 34.6 m. The reasoning hinges on separating horizontal and vertical motion; horizontal velocity stays constant.
Traps
- Mixing sign conventions mid-solution.
- Using v = 0 at the top for the whole velocity instead of only the vertical component.
- Forgetting that static friction adjusts up to μN; it is not always μN.
- Treating centripetal force as an extra outward force rather than the net inward force.