Physics: Mechanics, Circuits, Waves, and Thermodynamics
What the Physics section tests
The Physics domain samples mechanics, waves and optics, electricity and magnetism, and thermodynamics. Expect kinematics graphs, Newton's laws, work and energy, momentum, simple circuits, wave behavior, and the first law of thermodynamics. Items often require choosing the correct equation and tracking vector direction.
Core rules
- For constant acceleration, use v = u + at, s = ut + (1/2)at^2, and v^2 = u^2 + 2as. On velocity–time graphs, slope is acceleration and area is displacement.
- Newton's second law is a vector equation: sum F = ma. Resolve forces into components before summing; friction opposes relative motion.
- Energy is conserved if no non-conservative work acts: mgh + (1/2)mv^2 stays constant. Momentum is conserved in collisions when external forces are negligible.
- In circuits, Ohm's law V = IR applies to each resistor; series resistances add, parallel resistances combine as 1/R = 1/R1 + 1/R2. Power is P = IV.
- For waves, v = fλ; for light, Snell's law is n1 sin θ1 = n2 sin θ2. In thermodynamics, ΔU = Q - W with sign conventions defined by the course.
Worked example
A ball is thrown horizontally at 10 m/s from a 20 m cliff. Find the horizontal range (use g = 10 m/s²). Vertical motion starts from rest: h = (1/2)gt^2, so 20 = (1/2)(10)t^2, t^2 = 4, t = 2 s. Horizontal velocity is constant, so range = v_x t = 10 x 2 = 20 m. The reasoning separates vertical acceleration from horizontal uniform motion; mixing them is the usual error.
Traps that cost marks
Do not use g = 9.8 if the problem states 10, and do not drop signs for downward motion. In circuits, a parallel branch does not have the same current as the main line. In optics, measure angles from the normal, not the surface. Always carry units through the calculation.