Physics: Energy, Electricity, Magnetism, Motion, Light and Sound
Physics on the Tottori prefectural science paper asks you to connect an experiment diagram, a graph and a formula. The tested strands are 力学的エネルギー, 電気, 電流と磁界, 運動とエネルギー, 光と音. Expect short numerical answers and explanations of why a result changes when one condition is altered.
Energy and motion
With friction and air resistance ignored, mechanical energy is conserved: mgh + ½mv² is constant. A falling body converts height into speed; a pendulum converts back and forth. On a distance–time graph the slope is speed; on a velocity–time graph the slope is acceleration and the area is distance. Constant velocity means zero acceleration, not zero speed.
Circuits and magnetism
Ohm's law is V = RI. In series, current is common and resistances add; in parallel, voltage is common and currents add. Power is P = VI = RI². A coil carrying current creates a magnetic field; reversing the current reverses the field, and an iron core strengthens it. Electromagnetic induction requires relative motion between a conductor and a magnetic field. A motor changes electrical energy into kinetic energy; a generator does the reverse.
Light and sound
Reflection has equal angles. Refraction bends light toward the normal on entering a denser medium and away on leaving it. A convex lens gives a real, inverted image when the object is beyond the focal point. Sound needs a medium, travels fastest in solids, and has pitch set by frequency and loudness by amplitude.
Worked example
A 2.0 kg ball is released from rest 5.0 m above the ground. Find its speed just before impact; use g = 10 m/s². Reasoning: mgh = ½mv², so v = √(2 × 10 × 5) = √100 = 10 m/s. The mass cancels, so 2.0 kg is a distractor.
Traps
- Using g = 9.8 when the question specifies 10.
- Applying energy conservation when friction is present.
- Swapping voltage and current rules in parallel circuits.
- Reading the area under a distance–time graph as distance.
- Saying a generator creates energy rather than converting it.