Physics: Forces, Energy, and Circuits
What to master
ECAT physics rewards clean conceptual reasoning over memorised formulas. Be fluent with kinematics (equations of motion for constant acceleration), Newton's laws and free-body diagrams, work-energy and conservation of energy, momentum and collisions, circular motion, electric fields and potential, DC circuits (Ohm's law, series/parallel resistors, Kirchhoff's rules), and waves and optics (reflection, refraction, lenses).
Traps that catch candidates
- Confusing mass with weight and forgetting the direction of gravity in free-body diagrams.
- Using a kinematic equation that assumes constant acceleration when acceleration is changing.
- Sign errors in energy conservation: a falling object loses potential energy as it gains kinetic energy.
- In circuits, treating parallel resistors as adding directly; they add as reciprocals.
- Mixing speed with velocity and forgetting that momentum is a vector.
- Refraction problems where the angle is measured from the surface rather than the normal.
Rule of thumb
Always write the knowns with units, pick the equation that contains only one unknown, then check the dimensions and direction of the answer. If a result has the wrong sign or units, the arithmetic is wrong before the physics is.
Worked example
A 2 kg block slides from rest down a frictionless 5 m ramp inclined at 30°. Find its speed at the bottom. Height h = 5 sin30° = 2.5 m. Energy conservation: mgh = ½mv², so v = √(2gh) = √(2 × 9.8 × 2.5) ≈ 7.0 m/s. Note the mass cancels - a common shortcut trap.