Introductory Physics: Conservation Laws and Sign Sense
The tested core
Physics here is applied reasoning rather than derivation. Expect one- and two-dimensional kinematics, Newton's laws with friction and inclines, circular motion, work–energy, impulse–momentum, fluids (density, pressure, buoyancy, continuity, Bernoulli, Poiseuille), gas behavior and thermodynamics, electrostatics and circuits, magnetism, waves and sound (Doppler, beats, standing waves), geometric optics (Snell's law, thin lenses, mirrors), and a light touch of atomic and nuclear physics.
Traps
- Sign conventions: pick a positive direction once and stay with it. A decelerating object has acceleration opposite its velocity, not automatically a negative number.
- Reaching for kinematics when the force varies, or for energy when the question asks for a time.
- Mixing circuit rules: in series the current is shared (same value) and voltage divides; in parallel the voltage is shared and current divides.
- Treating Bernoulli as universal - it holds along a streamline in ideal, nonviscous flow.
- Unit slippage: cm to m, g to kg, and 1 N = 1 kg·m/s².
- Lens and mirror signs: diverging lenses have negative focal length and form virtual images.
Rule of thumb
When a stem gives a before-and-after (collision, explosion, spring release) and asks for a speed, conservation laws usually beat kinematics. Momentum is conserved in all collisions; kinetic energy only in elastic ones.
Worked example
A 2 kg block slides from rest down a frictionless 5 m ramp inclined 30°. The height drop is 5 sin 30° = 2.5 m, so v = √(2gh) = √(2 × 9.8 × 2.5) ≈ 7 m/s. Mass cancels - a shortcut worth remembering.