Physics: Motion, Forces, and Energy
Describing Motion
Speed is distance divided by time; velocity adds direction; acceleration is the change in velocity per unit time. On a distance-time graph, slope is speed; on a velocity-time graph, slope is acceleration and the area under the line is displacement. An object with constant velocity has zero acceleration even though it is moving, and an object changing direction is accelerating even at constant speed.
Forces and Newton's Laws
Newton's first law says an object stays at rest or in uniform motion unless a net force acts. The second law, F = ma, links net force, mass, and acceleration. The third law says forces come in equal and opposite pairs acting on different objects. Weight is a force (W = mg, with g about 9.8 N/kg on Earth) and differs from mass, which is constant. Friction opposes relative motion and converts kinetic energy to thermal energy.
Energy and Conservation
Energy is conserved but transforms between kinetic (KE = ½mv²), gravitational potential (PE = mgh), elastic, thermal, and other forms. Work is force times displacement in the direction of the force. Power is energy per time. A falling object trades potential for kinetic energy; a pendulum trades back and forth.
Traps and a Worked Example
Watch for questions that confuse mass with weight, ignore net force, or assume energy disappears to friction rather than becoming heat. A common trap is using total distance when displacement is asked. Worked example: a 2 kg ball falls 5 m. PE lost = mgh = 2 × 9.8 × 5 = 98 J, so KE at the bottom is 98 J and speed is about 9.9 m/s from KE = ½mv². Rule of thumb: track energy in and energy out before reaching for formulas.