Physics: Forces, Energy, Waves and Circuits
Physics on the Akita Entrance Science paper rewards clean setup: define the system, sketch the forces in words, then apply Newton's second law or a conservation law. Items often combine mechanics with graph reading or a simple circuit, so finish by checking units and significant figures.
Core rules and concepts
- Newton's second law: net force = mass × acceleration. Resolve forces along and perpendicular to the motion.
- Energy conservation: mgh + ½mv² + work done against friction = constant.
- For waves, v = fλ; for ohmic circuits, V = IR and P = IV.
- Dimensional consistency: both sides of any equation must reduce to the same base units.
Worked example
A 2.0 kg block starts from rest and slides 5.0 m down a frictionless incline at 30° to the horizontal. Find its speed at the bottom (g = 9.8 m/s²).
Reasoning: the vertical drop is h = 5.0 sin 30° = 2.5 m, not 5.0 m. The normal force is perpendicular to the motion, so it does no work. Energy conservation gives ½mv² = mgh, so v = √(2gh) = √(2 × 9.8 × 2.5) = √49 = 7.0 m/s. The mass cancels, which is a useful check.
Traps that cost marks
- Using the slope length as the height.
- Swapping sin and cos when resolving weight.
- Adding the normal force to the driving force.
- Reporting 7 m/s when the data justify two significant figures.