Physics,
one step at a time.

A free-body diagram contains external forces. The beam’s weight acts vertically downward through its centre of gravity; the support reaction acts upward, and the lamp pulls downward at its attachment point.

Watch for: Do not substitute numbers before stating the relationship used for Mechanics. Convert units first and explain what the calculated result means.

For an incremental extension dx, the work done is F dx. The total work, and hence elastic energy stored if losses are negligible, is the area under the force–extension curve; trapezia approximate this area.

Watch for: Do not substitute numbers before stating the relationship used for Materials. Convert units first and explain what the calculated result means.

A continuous classical wave model predicts that sufficient intensity or exposure time should eventually eject electrons at any frequency. The observed threshold frequency instead follows naturally from discrete photons of energy hf.

Watch for: Do not substitute numbers before stating the relationship used for Waves and Particle Nature of Light. Convert units first and explain what the calculated result means.

In I = nqvA, electron charge q is fixed, but carrier density n and the drift velocity produced by a given electric field depend on the material and scattering. These microscopic differences produce different resistivities.

Watch for: Do not substitute numbers before stating the relationship used for Electric Circuits. Convert units first and explain what the calculated result means.

Velocity is a vector. Although its magnitude is constant, its direction changes, producing centripetal acceleration; the resultant force is therefore directed toward the centre.

Watch for: Do not substitute numbers before stating the relationship used for Further Mechanics. Convert units first and explain what the calculated result means.

During switching, the primary current and its magnetic field change. The changing flux linkage through the secondary induces an e.m.f.; once the primary current is steady, the flux is steady and the induced e.m.f. becomes zero.

Watch for: Do not substitute numbers before stating the relationship used for Electric and Magnetic Fields. Convert units first and explain what the calculated result means.

Large-angle deflections require a strong, concentrated repulsive field, showing that positive charge and mass are concentrated in a very small nucleus. Most particles pass through because atoms are mostly empty space.

Watch for: Do not substitute numbers before stating the relationship used for Nuclear and Particle Physics. Convert units first and explain what the calculated result means.

Internal energy is distributed microscopically among random particle kinetic energies and intermolecular potential energies. It excludes ordered motion of the whole body and external gravitational energy.

Watch for: Do not substitute numbers before stating the relationship used for Thermodynamics. Convert units first and explain what the calculated result means.

Positively charged nuclei repel through the Coulomb force. High temperature provides kinetic energy and high density increases collision rate, allowing some nuclei to approach close enough for the attractive strong interaction to bind them.

Watch for: Do not substitute numbers before stating the relationship used for Nuclear Decay. Convert units first and explain what the calculated result means.

SHM requires restoring acceleration, and therefore resultant force for constant mass, proportional to displacement and opposite in direction. The minus sign represents the restoring direction.

Watch for: Do not substitute numbers before stating the relationship used for Oscillations. Convert units first and explain what the calculated result means.

An ideal black body is a perfect absorber. By thermal-radiation principles it also has maximum emissive power for its temperature, with a continuous spectrum whose shape depends on temperature.

Watch for: Do not substitute numbers before stating the relationship used for Astrophysics and Cosmology. Convert units first and explain what the calculated result means.

A longer fall produces a longer measured time, making a fixed absolute timing resolution a smaller fraction of the value. The distance should still be measured accurately and air resistance considered.

Watch for: Do not substitute numbers before stating the relationship used for Core practical 1: determine the acceleration of a freely-falling object. Convert units first and explain what the calculated result means.

For liquids, viscosity commonly decreases as temperature rises. Uncontrolled warming therefore changes the measured terminal speed even if all geometric variables are fixed.

Watch for: Do not substitute numbers before stating the relationship used for Core practical 2: use a falling-ball method to determine the viscosity of a liquid. Convert units first and explain what the calculated result means.

The wire may vary along its length or be slightly oval. Repeated measurements in different places and orientations quantify this scatter and improve the area estimate.

Watch for: Do not substitute numbers before stating the relationship used for Core practical 3: determine the Young modulus of a material. Convert units first and explain what the calculated result means.

Maintaining alignment controls geometry and signal quality. Moving sideways could change detected amplitude and reflected-path contributions, making phase matching less reliable.

Watch for: Do not substitute numbers before stating the relationship used for Core practical 4: determine the speed of sound in air using a two-beam oscilloscope. Convert units first and explain what the calculated result means.

A slow sweep resolves the peak more finely, repeats reveal judgement scatter, and approaching from both directions helps detect any systematic lag or hysteresis.

Watch for: Do not substitute numbers before stating the relationship used for Core practical 5: investigate how length, tension and mass per unit length affect a vibrating string. Convert units first and explain what the calculated result means.

A single monochromatic source has one wavelength. Order-dependent results indicate a method, identification or alignment problem rather than a physical change of wavelength.

Watch for: Do not substitute numbers before stating the relationship used for Core practical 6: determine the wavelength of light using a diffraction grating. Convert units first and explain what the calculated result means.

A low-resistance path can draw excessive current, heat components and discharge the cell. Current limiting and brief connection protect apparatus and keep temperature controlled.

Watch for: Do not substitute numbers before stating the relationship used for Core practical 7: determine the electrical resistivity of a material. Convert units first and explain what the calculated result means.

With nearly zero current, the internal potential drop Ir is nearly zero. The terminal p.d. therefore approaches energy supplied per charge, the e.m.f.

Watch for: Do not substitute numbers before stating the relationship used for Core practical 8: determine the e.m.f. and internal resistance of an electrical cell. Convert units first and explain what the calculated result means.

The hanging weight is not all available for acceleration when resistive forces act. The resultant force is weight minus resistance, so using weight alone overestimates impulse.

Watch for: Do not substitute numbers before stating the relationship used for Core practical 9: investigate force and change of momentum. Convert units first and explain what the calculated result means.

Apparent curvature may arise from imaging geometry or real acceleration on a sloping table. Both must be controlled before collision momentum is interpreted.

Watch for: Do not substitute numbers before stating the relationship used for Core practical 10: use ICT to analyse two-dimensional collisions. Convert units first and explain what the calculated result means.

A suitable square wave repeatedly switches the input, allowing identical transients to be superimposed and compared. The period must still be long enough for substantial charging and discharging.

Watch for: Do not substitute numbers before stating the relationship used for Core practical 11: analyse capacitor charging and discharging with an oscilloscope or data logger. Convert units first and explain what the calculated result means.

Bracketing the operating point provides evidence for the local response and reduces uncertainty compared with extrapolation outside the observed range.

Watch for: Do not substitute numbers before stating the relationship used for Core practical 12: calibrate a thermistor potential divider as a thermostat. Convert units first and explain what the calculated result means.

Electrical heating combines current, hot components and possible liquid. Low-voltage supplies, dry insulated connections and stable heat-resistant containers control these hazards.

Watch for: Do not substitute numbers before stating the relationship used for Core practical 13: determine the specific latent heat of a phase change. Convert units first and explain what the calculated result means.

Boyle’s relationship requires constant temperature. Compression can temporarily increase internal energy and temperature, so equilibrium time reduces this systematic departure.

Watch for: Do not substitute numbers before stating the relationship used for Core practical 14: investigate pressure and volume of a gas at fixed temperature. Convert units first and explain what the calculated result means.

Environmental and detector-background events contribute even with the source absent. Their mean rate must be removed from each source-plus-background rate.

Watch for: Do not substitute numbers before stating the relationship used for Core practical 15: investigate absorption of gamma radiation by lead. Convert units first and explain what the calculated result means.

The oscillator includes the hanger and part of the spring. Their effective constant mass shifts the calibration even when added mass is zero.

Watch for: Do not substitute numbers before stating the relationship used for Core practical 16: determine an unknown mass from resonant frequencies. Convert units first and explain what the calculated result means.