Practice Test 4 - High School Physics

20 questions. Answer them in any order, then submit to see your score and every explanation.

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1. A metal spoon and a wooden stir stick are placed with their handles sticking out of a mug of hot water (about 80°C). After 1 minute, the metal spoon's handle feels much warmer than the wooden handle. Which thermal energy transfer mechanism is primarily responsible for the heat reaching the end of the metal handle? (Recall: conduction is heat transfer through direct contact within a material via molecular collisions/vibrations with no bulk motion; convection is heat transfer by moving fluid; radiation is heat transfer by electromagnetic waves and can occur across a vacuum.)
2. A heat engine receives $E_{in} = 5000\ \text{J}$ of thermal energy from burning fuel and produces $E_{out} = 1500\ \text{J}$ of useful mechanical work. The rest is expelled as waste heat. What percentage of the input energy is wasted as thermal energy? Use: $E_{lost}=E_{in}-E_{out}$ and $\%\text{ wasted} = \left(\frac{E_{lost}}{E_{in}}\right)\times 100\%$.
3. In a lab, a 200-turn coil is connected to a sensitive galvanometer (forming a closed circuit). A student quickly pushes the north pole of a bar magnet straight into the center of the coil and then holds the magnet completely still inside the coil. Which observation best indicates when electromagnetic induction occurs in this setup (Faraday's law: $|\varepsilon| = N\left|\Delta\Phi/\Delta t\right|$)?
4. A football helmet liner is being chosen. Both liners have the same thickness and fit constraints, but one is designed to undergo mostly plastic deformation (crush) while the other behaves more elastically (springs back). To best reduce injury risk in a single severe impact, which liner behavior is preferred and why (using energy absorption ideas)?
5. A seismic P-wave travels through rock with speed $v = 6.0\ \text{km/s}$ and has wavelength $\lambda = 3.0\ \text{km}$. What is its frequency $f$? Use $f = v/\lambda$.
6. A clean metal plate is illuminated with light. Electrons are emitted only when the light frequency is above a certain threshold, and increasing the intensity (brightness) above threshold increases the number of emitted electrons but not their maximum kinetic energy. Which model best explains these observations?
7. An electron microscope can achieve much higher resolution than a visible-light microscope. Which statement best connects this to wave-particle duality? ​
8. In a 1D collision investigation, students measure $m_1$, $m_2$, $v_{1i}$, $v_{2i}$, $v_{1f}$, and $v_{2f}$. Which calculation should they perform to test the hypothesis that total momentum is conserved?
9. A $5.0\,\text{kg}$ sled slides down a frictionless incline and speeds up from $2.0\,\text{m/s}$ to $10.0\,\text{m/s}$. What is the net work done on the sled during this motion? (Include sign.)
10. An airbag system is designed to reduce the average force on a $60\,\text{kg}$ driver in a $v=18\,\text{m/s}$ frontal crash by increasing the stopping time $\Delta t$. The safety requirement is $F \le 2500\,\text{N}$. The design parameter is airbag-induced stopping time (assume the driver comes to rest over $\Delta t$). Constraint: the airbag can only maintain cushioning for up to $0.50\,\text{s}$ before bottoming out. What minimum $\Delta t$ is required, and does the constraint allow it?
11. A $10\,\text{kg}$ box rests on a $20^\circ$ incline. It is held at rest by static friction (no rope). Take $g=10\,\text{m/s}^2$. The component of weight parallel to the incline points down the slope. In which direction does the static friction force on the box act?
12. A straight horizontal wire segment of length $L$ lies on the lab bench and carries current $I$ to the right (→). The wire is placed in a uniform external magnetic field $\vec{B}$ directed into the page (⊗) to investigate the force of a magnetic field on a current. What is the direction of the magnetic force $\vec{F}$ on the wire segment?
13. A geostationary satellite of mass $1.0 \times 10^3\ \text{kg}$ orbits Earth at an altitude of $3.6 \times 10^7\ \text{m}$ above the surface. Use $M_E = 6.0 \times 10^{24}\ \text{kg}$, $R_E = 6.4 \times 10^6\ \text{m}$, and $G = 6.67 \times 10^{-11}\ \text{N}\cdot\text{m}^2/\text{kg}^2$. What is the magnitude of the gravitational force on the satellite? (Use $r = R_E + \text{altitude}$.)
14. Two identical mugs contain the same amount of water at 80°C in a 20°C room. Mug 1 is wrapped in a thick foam sleeve; Mug 2 is not wrapped. After 10 minutes, Mug 1 is warmer than Mug 2. Which statement best explains why the foam sleeve is effective?
15. Two point charges are separated by a distance $r$. If $q_1$ is doubled while $q_2$ and $r$ stay the same, by what factor does the magnitude of the electric force change? (Use Coulomb's Law $F = k|q_1 q_2|/r^2$.)
16. Which phenomenon is best explained by the particle (photon) model of light rather than the wave model?
17. Two different experiments are performed: 1) Light passing through two slits forms bright and dark fringes on a screen. 2) Light shining on a metal ejects electrons only above a threshold frequency. Which pairing correctly matches each experiment with the model that best explains it?
18. A sound wave in air has energy proportional to amplitude squared: $E \propto A^2$. A technician wants to double the energy output ($E_2 = 2E_1$) without changing frequency. By what factor must the amplitude change ($A_2/A_1$)?
19. A beam of light passes through a polarizing filter. When a second polarizer is rotated relative to the first, the transmitted intensity changes and can reach nearly zero at a 90° rotation. Which model best explains why rotating the filter changes the transmitted intensity?​
20. Light of wavelength $\lambda$ is incident on a surface and transfers momentum to it (radiation pressure). Which statement best connects this observation to a model of light?

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