Physics: Modeling Motion, Forces, Circuits, and Waves
Physics on this exam checks whether you can translate a described situation into equations, solve with consistent units, and interpret the result physically. The core territory is mechanics, electric circuits, waves, and thermal or nuclear energy.
Mechanics and energy
- Use SI units throughout. Convert grams to kilograms, centimeters to meters, and minutes to seconds before substituting.
- Resolve vectors into perpendicular components. On an incline, the component of gravity along the slope is $mg\sin\theta$ and the normal force is $mg\cos\theta$ when no other vertical force acts.
- Apply Newton's second law as $F_{\text{net}}=ma$, not to individual forces in isolation. Static friction has a maximum value $\mu_s N$; kinetic friction is usually modeled as $\mu_k N$.
- Use conservation laws when forces are conservative: $mgh+\tfrac12mv^2$ is constant. Work done by friction removes mechanical energy.
- For circuits, Ohm's law is $V=IR$. Series resistances add; parallel branches share voltage and add as reciprocals. Internal resistance reduces terminal voltage under load.
- Wave speed is $v=f\lambda$. Period and frequency are reciprocals. For a half-life $t_{1/2}$, the remaining fraction after time $t$ is $(1/2)^{t/t_{1/2}}$.
Worked example
A 2.0 kg block starts from rest and slides 5.0 m down a frictionless ramp inclined at 30°. Find its speed at the bottom.
The vertical drop is $h=5.0\sin30^\circ=2.5$ m. Mechanical energy is conserved: $mgh=\tfrac12mv^2$. Mass cancels, so $v=\sqrt{2gh}=\sqrt{2(9.8)(2.5)}\approx7.0$ m/s. If friction were present, subtract $f_k d$ from $mgh$ before solving.
Traps that cost marks
- Treating weight in newtons as mass in kilograms.
- Using degrees in a formula that expects radians, or omitting the sine/cosine component on a ramp.
- Assuming normal force always equals $mg$; it does not on an incline or in an accelerating elevator.
- Adding parallel resistances directly instead of using reciprocals.
- Reporting more significant figures than the data justify.