Physics Content Knowledge
Scope of the domain
Physics content knowledge for Grades 9–12 is assessed here as one content domain. Items may draw on mechanics, waves, optics, electricity and magnetism, thermal physics, and modern physics. Quantitative questions require selecting a model, setting up an equation, and checking units and limiting behavior. Qualitative questions ask why a principle applies, not merely for a remembered phrase. A correct response often depends on a sign convention or a model assumption.
Core rules
- Newton's second law: ΣF=ma; weight is mg, while mass is invariant.
- Conservation of energy: account for kinetic, potential, and internal energy; work is Fd cosθ.
- Momentum and impulse: J=FΔt=Δp; momentum is conserved in an isolated system, and collisions may be elastic or inelastic.
- Circular motion: the net inward force supplies centripetal acceleration a=v²/r; it is not a new force.
- Waves: v=fλ; wave speed is set by the medium, while frequency is set by the source.
- Optics: the reflection angle equals the incidence angle; Snell's law is n1 sinθ1=n2 sinθ2.
- Circuits: series current is common; parallel voltage is common; P=IV=I²R.
- Thermal physics: heat is energy transfer; temperature measures average molecular kinetic energy.
- Modern physics: photon energy E=hf; photoelectric emission requires frequency above a threshold.
Worked example
A 0.50 kg cart at rest is pushed by a constant 2.0 N net force for 3.0 s. Find its final speed and distance traveled. Reasoning: a=F/m=2.0/0.50=4.0 m/s². Then v=at=4.0×3.0=12 m/s. Distance d=½at²=0.5×4.0×9.0=18 m. Check with work–energy: Fd=2.0×18=36 J; ½mv²=0.5×0.50×144=36 J.
Traps that cost marks
- Confusing mass with weight; the newton is a force unit.
- Using degrees in formulas requiring radians, such as s=rθ or ω=2π/T derivations.
- Saying current is "used up" in series; charge is conserved and current is the same.
- Letting source frequency change wave speed when a wave enters a new medium.
- Treating heat and temperature as identical, or ignoring sign conventions for work and heat.
- Assuming a real image is always upright or that a virtual image cannot be seen.