Physics: Models, Conservation Laws, and Units
Core concepts
Build fluency in kinematics and Newton's laws, work-energy and power, momentum and collisions, circular and rotational motion, oscillations and waves, sound and light, thermal physics, electrostatics, current electricity, magnetism, electromagnetic induction, and modern physics. For each topic, identify the conserved quantity: energy in conservative systems, momentum in collisions, charge in circuits, and mass-energy in nuclear reactions. Practice translating word problems into equations with consistent units, and sketch free-body diagrams before applying Newton's second law.
Exam traps
Sign conventions cause many errors: fix a positive direction and keep it. Distinguish distance from displacement, speed from velocity, and mass from weight. In projectile motion, vertical velocity is zero at the highest point, but horizontal velocity is not. In circuits, series resistors share current, while parallel resistors share voltage. In waves, frequency is set by the source and does not change when the medium changes; wavelength and speed do.
Worked example
Two resistors, 6 Ω and 3 Ω, are connected in parallel across a 12 V battery. The equivalent resistance is (6×3)/(6+3)=2 Ω. The total current is I=V/R=12/2=6 A. The 6 Ω resistor carries 2 A and the 3 Ω resistor carries 4 A, confirming that the larger current flows through the smaller resistance.
Rule of thumb
Draw the diagram, list knowns with units, and solve symbolically before substituting numbers.