Physics: measurements, energy and circuits
Begin a calculation by defining the system and converting units. Average speed is total distance divided by elapsed time, whereas average velocity uses displacement and includes direction. The signed area under a velocity-time graph gives displacement. To obtain total distance, add the magnitudes of areas where velocity is positive or negative. A horizontal line on a speed-time graph means constant speed, not zero speed. Acceleration describes velocity change, so turning can involve acceleration even when speed stays constant.
Draw forces on one object before applying a motion rule. Balanced forces on a stationary book are not an action-reaction pair: the two members of such a pair act on different objects. For a constant force parallel to displacement, work is force times displacement. A frictionless sliding particle can exchange gravitational potential energy mgh with kinetic energy mv²/2. A rolling object can also have rotational kinetic energy, so do not silently use the sliding model for it. State the reference height and whether friction or other energy transfers matter.
In a simple ohmic circuit at constant physical conditions, V=IR. Check the graph axes before interpreting its slope: an I-against-V slope is 1/R. Series components share current, while ideal parallel branches share voltage. Electrical energy is power times time, and watt-hours must be converted when joules are requested. For waves, speed equals frequency times wavelength. Refraction depends on optical refractive index and incidence conditions, not a general rule about mass density. Distinguish an observation from the ideal model used to explain it.