Physics: mechanics, forces, energy, electricity and magnetism, waves
What the physics section asks
The Saga entrance science paper tests mechanics, forces, energy, electricity and magnetism, and waves. Questions usually combine a calculation with a graph or circuit diagram, so you must state the rule, substitute SI units, and give the direction or unit.
Core rules
- Forces and motion: A resultant force changes velocity; with zero resultant force, an object stays at rest or moves at constant velocity. Use F = ma and weight W = mg, with g ≈ 9.8 N/kg near Earth.
- Energy: Total energy is conserved. Gravitational potential energy is mgh, kinetic energy is ½mv², and work is force × distance moved in the force direction. Efficiency = useful output ÷ total input × 100%.
- Electricity and magnetism: Ohm's law V = IR. In series, current is the same and voltages add; in parallel, voltage is the same and currents add. Power P = VI = I²R. A current-carrying wire has a magnetic field; in a magnetic field it can experience a force.
- Waves: Wave speed v = fλ. Reflection has angle of incidence equal to angle of reflection; refraction occurs when speed changes; sound requires a medium.
Worked example
A 2.0 kg ball is released from rest and falls 5.0 m. Ignoring air resistance, find its speed just before impact. Loss of gravitational potential energy = mgh = 2.0 × 9.8 × 5.0 = 98 J. This becomes kinetic energy: ½mv² = 98, so v² = 98/1.0 = 98, and v ≈ 9.9 m/s. The mass cancels; doubling the mass would not change the speed.
Traps
Using grams instead of kilograms; confusing mass with weight; treating speed as velocity when direction matters; adding resistances directly in parallel circuits; forgetting that the force on a current-carrying wire is zero when the wire is parallel to the external magnetic field; ignoring significant figures or units in the final answer.