Forces, Motion, Energy, and Waves
What to know
Newton's laws anchor mechanics. First law: an object at rest or in uniform motion stays that way unless a net force acts (inertia). Second law: F = ma, so acceleration is proportional to net force and inversely proportional to mass. Third law: forces come in equal, opposite pairs acting on different objects. Weight is mg, distinct from mass. Friction opposes relative motion; gravity near Earth accelerates objects at about 9.8 m/s² downward, independent of mass in free fall.
Kinematics: speed is distance/time; velocity includes direction; acceleration is change in velocity per time. On a distance–time graph, slope is speed; on a velocity–time graph, slope is acceleration and area under the curve is displacement.
Energy is conserved but transforms among kinetic (½mv²), potential (mgh), thermal, chemical, and electrical forms. Work = force × distance in the force direction. Power = work/time. Simple machines trade force for distance without reducing total work.
Waves transfer energy, not matter. Know wavelength, frequency, amplitude, and speed (v = fλ). Transverse waves oscillate perpendicular to travel; longitudinal waves (sound) oscillate parallel. Sound needs a medium; light does not. Reflection, refraction, diffraction, and interference are core behaviors.
Traps
Questions mix up mass and weight, confuse velocity with acceleration, or assume a constant-speed object has zero net force but also zero forces. A common error is treating action–reaction pairs as canceling on the same object.
Rule of thumb and worked example
A 2 kg cart accelerates at 3 m/s². Net force = ma = 2 × 3 = 6 N. If a 12 N force pushes a 4 kg block with 4 N of friction, net force is 8 N, so a = 8/4 = 2 m/s².