Physics: Forces, Motion, Energy, Light and Sound
What this domain tests
The physics items on the Kagoshima entrance paper ask you to read motion off graphs, apply Newton's laws and energy conservation, and explain everyday optical and acoustic events. The calculations are short; the marks sit in units, direction, and in explaining why something happens rather than just quoting a number.
Motion
- Speed = distance ÷ time; velocity is speed with direction. Acceleration = change in velocity ÷ time taken.
- On a velocity–time graph the slope gives acceleration and the area under the line gives distance travelled.
- Newton's first law: with no resultant force, motion does not change. Second law: F = ma, where F is the resultant force. Third law: equal and opposite forces acting on two different objects.
- Free fall near the ground: about 9.8 m/s² downward, the same for heavy and light objects when air resistance is negligible.
Energy
Work done = force × distance moved along the force, in joules. Kinetic energy = ½mv²; gravitational potential energy = mgh. Without friction, mechanical energy is conserved; with friction some becomes heat, and efficiency = useful output ÷ total input × 100%.
Light and sound
Reflection: angle of incidence equals angle of reflection, both measured from the normal. Refraction into a denser medium bends light toward the normal and slows it. Sound is a longitudinal wave that needs a medium; v = fλ, roughly 340 m/s in air, frequency giving pitch and amplitude giving loudness.
Worked example
A 2.0 kg cart moving at 3.0 m/s runs up a frictionless slope. How high does it get? Kinetic energy = ½ × 2.0 × 3.0² = 9.0 J. All of it becomes mgh: 9.0 = 2.0 × 9.8 × h, so h ≈ 0.46 m. The mass cancels, giving h = v²/2g - a useful check.
Traps that cost marks
Using the final speed instead of the area under a speed–time graph to find distance. Mixing mass (kg) with weight (N). Giving energy in newtons. Measuring refraction angles from the water surface rather than the normal. Saying sound can cross a vacuum.