Practice Test 9 - High School Physics

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

A student claims: "If light is a wave, then any frequency of light should eject electrons from a metal as long as the intensity is high enough." In the photoelectric effect, what observation shows this claim is incorrect?
A student claims: "If we just add more amplifiers, an analog signal can be sent any distance with no loss, just like digital." The channel adds some random noise on each segment, and each analog amplifier boosts whatever it receives. Which response best evaluates the claim?
Two objects in space have masses $m_1 = 1.0 \times 10^{5}\ \text{kg}$ and $m_2 = 1.0 \times 10^{5}\ \text{kg}$ . Their center-to-center separation is reduced from $r = 100\ \text{m}$ to $r = 50\ \text{m}$ . By what factor does the gravitational force change? (Use $F \propto 1/r^2$ .)
A student looks at two stored versions of the same lecture: (1) an analog recording on magnetic tape, and (2) a digital recording saved as a file. The student plans to make many duplicates for classmates. Which option is the best choice and why?
Two rods of the same length are heated at one end with the same flame: one rod is copper and the other is plastic. The far end of the copper rod becomes hot much sooner than the far end of the plastic rod. This observation most strongly indicates that which mechanism is dominating heat transfer along the rods?
A $1000\,\text{kg}$ car traveling at $22\,\text{m/s}$ is being redesigned. Engineers can choose either to (i) increase crumple distance $d$ or (ii) accept the current $d=0.70\,\text{m}$ . The safety requirement is $F \le 4.0\times 10^5\,\text{N}$ using $F=\text{KE}/d$ . Constraint: due to styling, $d$ cannot exceed $0.80\,\text{m}$ . What minimum $d$ is required, and what design conclusion follows?
Two pulses travel along the same stretched rope (same tension and frequency). Pulse 1 has amplitude $A_1 = 2.0\ \text{cm}$ and Pulse 2 has amplitude $A_2 = 4.0\ \text{cm}$ . If wave energy on a rope follows $E \propto A^2$ , what is the ratio $E_2/E_1$ ?
A homeowner is considering a solar water-heating panel that converts solar radiation → thermal energy for domestic hot water. Assume sunlight intensity is $1000\ \text{W/m}^2$ and the collector converts $\eta=50\%$ of incident solar power into useful heat. The system must supply at least $P_{thermal} \ge 800\ \text{W}$ of heating power at noon. The collector area must be $\le 2.0\ \text{m}^2$ . If the panel is installed at an angle where the sunlight hits it at $\theta=60^\circ$ from perpendicular, so effective area is $A_{eff}=A\cos\theta$ , what minimum actual panel area $A$ is required?
A circular coil is connected to a galvanometer. A bar magnet's north pole is moved toward the coil along its central axis. When the magnet is pushed toward the coil, the galvanometer deflects to the right. If the student then pulls the magnet straight back out along the same axis at the same speed, what should happen to the galvanometer deflection, and why?
A straight horizontal wire segment of length $L=0.10\,\text{m}$ carries current $I=3.0\,\text{A}$ to the right (→). The wire is placed in a uniform external magnetic field $\vec{B}$ directed into the page (⊗) to investigate how a magnetic field affects a current. What is the direction of the magnetic force on the wire segment?
For a momentum conservation test with two carts on a track, students will repeat the same collision several times. They want to reduce uncertainty so that $|p_{\text{after}}-p_{\text{before}}|/p_{\text{before}} \times 100\%$ is as small as possible. Which change would most directly improve the reliability/precision of the momentum comparison?
A 1500 kg car (Object A) and a 1500 kg car (Object B) collide on an icy straight road (rightward positive). Before the collision, Object A moves at $v_{A i} = +6.0\,\text{m/s}$ and Object B moves at $v_{B i} = -4.0\,\text{m/s}$ . After the collision (elastic), Object A moves at $v_{A f} = -4.0\,\text{m/s}$ . Using conservation of momentum, what is Object B's final velocity $v_{B f}$ ?
A bicycle helmet must limit the wearer's head deceleration to less than $100\ g$ during an impact. A test headform of mass $m=5.0\ \text{kg}$ hits a barrier at $v_i=6.0\ \text{m/s}$ and comes to rest. Assume the helmet padding provides an approximately constant stopping distance equal to its compressible thickness $d$ (ignore shell deformation). Helmet options: - Design A: soft foam, $d=1\ \text{cm}=0.01\ \text{m}$ . - Design B: multi-density foam, $d=3\ \text{cm}=0.03\ \text{m}$ . - Design C: air-pocket liner, $d=5\ \text{cm}=0.05\ \text{m}$ . - Design D: minimal liner, $d=0.5\ \text{cm}=0.005\ \text{m}$ . Using $v^2\approx 2ad$ (constant deceleration), which design(s) meet the requirement $a<100g$ ? Select the best answer.
Two identical pith balls are charged and hung so they repel each other. Treat them as point charges with $q_1 = +3.0\,\mu\text{C}$ and $q_2 = +1.0\,\mu\text{C}$ . Their centers are $r = 25\,\text{cm}$ apart. Using $k = 9.0\times10^9\,\text{N}\cdot\text{m}^2/\text{C}^2$ , what is the magnitude of the electric force between them (in N)?
Two charged spheres are $r = 0.20\,\text{m}$ apart. Initially, $q_1 = +3.0\,\mu\text{C}$ and $q_2 = +4.0\,\mu\text{C}$ . Without changing the distance, $q_1$ is doubled to $+6.0\,\mu\text{C}$ while $q_2$ stays the same. By what factor does the electric force magnitude change?
Two objects are separated by the same distance $r$ . The gravitational force is $F_{\text{grav}} = Gm_1m_2/r^2$ and the electric force is $F_{\text{elec}} = kq_1q_2/r^2$ . If the distance between the objects is doubled to $2r$ , how do $F_{\text{grav}}$ and $F_{\text{elec}}$ change, and what happens to the ratio $F_{\text{elec}}/F_{\text{grav}}$ ?
To investigate momentum conservation in a 1D collision, you measure masses with a balance and velocities with motion sensors. During analysis, you must assign signs to velocities (e.g., rightward positive, leftward negative). Which calculation correctly represents the total momentum after the collision for two carts? Let $m_1, m_2$ be in kg and $v_{1f}, v_{2f}$ be in m/s.
A rectangular coil rotates at constant speed between the poles of a magnet (a simple generator). The magnetic flux through the coil is $\Phi = BA\cos(\theta)$ , where $\theta$ is the angle between the magnetic field and the coil's area normal. At which orientation is the magnitude of the induced EMF greatest?
A lab tests transmission over the same 1 km cable. For analog voice, intelligibility becomes poor below about 40 dB SNR. A digital voice link with error correction can still work around 15–20 dB SNR, but fails if SNR gets too low to distinguish 0 from 1 reliably. If the measured SNR is 18 dB, which result is most likely?
In a 1D elastic collision on a frictionless track, two equal-mass carts collide head-on. Cart A has mass $m_A = 1.0\,\text{kg}$ and initial velocity $v_{A i} = +4.0\,\text{m/s}$ . Cart B has mass $m_B = 1.0\,\text{kg}$ and initial velocity $v_{B i} = -1.0\,\text{m/s}$ . After the collision, Cart A is observed moving at $v_{A f} = -1.0\,\text{m/s}$ . Using conservation of momentum, what is Cart B's final velocity $v_{B f}$ (in m/s)?

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