Physics case studies
Physics · Put your knowledge to work in context.
The Physics exam is a standardized, multiple-choice subject test used in undergraduate admissions and placement for physics and engineering programs. It is aimed at students who have completed a solid introductory physics sequence, typically one year of algebra- or calculus-based physics at the level of a first university course. The Physics case studies are 5 cases and about 3,825 words.
1Loop-the-loop: is the ball a point particle or a rolling body?
A solid steel ball of radius 8.0 mm and mass 5.2 g is released from rest and rolls without slipping down a straight ramp that joins a vertical circular track of radius R = 0.35 m tangentially. Students bracket the minimum release height, measured from the track's lowest point to the ball's centre, by trial: the ball stays in contact through the loop when released from 0.98 m, but leaves the track before reaching the top when released from 0.90 m.
798 words, about 4 min2Magnet falling through a copper tube: forces, energy and a slit
A cylindrical neodymium magnet, 10 mm in diameter and of mass 12.0 g, is dropped from rest into a vertical copper tube 1.00 m long with a 14 mm bore. Phone slow-motion video gives fall times of 2.35 s, 2.40 s and 2.45 s.
742 words, about 4 min3Resonance tube: which value of the speed of sound should be quoted?
A student measures the speed of sound with a resonance tube: a plastic tube of internal diameter 4.0 cm, closed by a movable piston and driven by a loudspeaker at 512 Hz. The loudest resonances occur with the piston at 16.0 cm and 49.5 cm from the open end.
713 words, about 4 min4Gas-thermometer extrapolation: a straight line to the wrong absolute zero
A student traps dry air in a sealed 250 mL glass flask joined to a pressure sensor by 30 cm of rubber tubing, and records the pressure with the flask in a stirred water bath: 0.0 °C gives 100.0 kPa, 20.0 °C gives 107.0 kPa, 40.0 °C gives 113.9 kPa, 60.0 °C gives 120.8 kPa, 80.0 °C gives 127.7 kPa. The sensor is accurate to plus or minus 0.3 kPa and the bath to plus or minus 0.2 °C.
789 words, about 4 min5Photoelectric effect: trusting the gradient but not the intercept
A student uses a vacuum photocell with a sodium cathode and a set of interference filters, measuring the stopping potential by increasing the reverse voltage until the photocurrent falls to zero: 365 nm gives 1.22 V, 405 nm gives 0.89 V, 436 nm gives 0.67 V, 480 nm gives 0.41 V, and at 546 nm no current is detected at any lamp intensity. A least-squares fit of stopping potential against frequency gives a gradient of 4.10 × 10⁻¹⁵ V s and an intercept of −2.15 V.
783 words, about 4 min