Physics - Mie Entrance Science
The Physics section tests mechanics, waves, electricity and magnetism, thermodynamics, and atomic physics. Questions reward a correct physical model, clear sign conventions, and unit-consistent algebra more than memorized formulas.
Core rules
- Mechanics: draw a free-body diagram before resolving forces; apply F = ma along each axis. Energy conservation applies when non-conservative work is zero; momentum is conserved in collisions without external impulse.
- Circular motion: centripetal force is the net inward force, not an extra force. At the top of a vertical loop, gravity and normal force both point downward.
- Waves: standing waves have nodes where displacement is always zero; a string fixed at both ends has fundamental wavelength 2L.
- Circuits: Kirchhoff’s junction rule is charge conservation; the loop rule is energy per charge. Parallel resistors share voltage; series resistors share current.
- Thermodynamics: use the first law with a consistent sign convention, and the ideal gas law PV = nRT.
Worked example
A block is released from rest at height h on a frictionless track and must complete a vertical circular loop of radius R. At the loop’s top, mg + N = mv²/R; staying on the track requires N ≥ 0, so v² ≥ gR. Energy from release to top gives mgh = mg(2R) + ½mv². Substituting v² = gR yields h ≥ 2R + R/2 = 2.5R.
Traps
- Setting v = 0 at the top of a loop; the block would lose contact earlier.
- Forgetting that friction does negative work and changes mechanical energy.
- Mixing cm and m, or grams and kilograms, in one equation.
- Reversing the sign of heat or work in the first law.
- Treating static friction as always equal to μsN; it only reaches that maximum at slipping.