Physics - 力と運動・電流・光と音・エネルギー
力と運動
Draw forces as arrows: length shows magnitude, plus direction and point of application. Gravity acts downward from the centre of gravity; the normal force is perpendicular to the contact surface; friction opposes relative sliding; tension pulls along a string. Forces balance only when they are equal in magnitude, opposite in direction and along the same line. Action–reaction pairs are equal and opposite but act on different objects, so they never cancel each other - a favourite trap. On a distance–time graph the slope gives speed: horizontal means at rest, a straight slope means uniform motion, a curve means changing speed.
電流
Ohm's law gives V = RI. In series the current is identical everywhere, voltages add and resistances add. In parallel the voltage is identical, currents add and 1/R = 1/R₁ + 1/R₂, so the combined resistance is smaller than any single branch. Power is P = VI and electrical energy is Pt (J).
光と音
The angle of incidence equals the angle of reflection, both measured from the normal. Light entering a denser medium bends toward the normal; a convex lens brings parallel rays to its focal point. Sound is produced by vibration, needs a medium (never a vacuum) and travels at about 340 m/s in air; frequency fixes pitch and amplitude fixes loudness.
エネルギー
Work = force × distance moved along the force. Kinetic energy grows with the square of speed; gravitational potential energy is mgh. Energy is conserved overall but is degraded into heat.
Rule of thumb. A clap echoes back after 1.2 s; with sound at 340 m/s the distance to the cliff is 340 × 1.2 ÷ 2 = 204 m. Halve echo times, and convert cm to m, g to kg and mA to A before substituting.