IISER IAT Physics: Models, Signs, and Energy
Model-first problem solving
IISER IAT Physics rewards choosing the right model before substituting numbers. Mechanics questions usually yield to energy or momentum conservation; circuits to Kirchhoff and equivalent resistance; optics to the lens/mirror sign convention; thermodynamics to the first law and process-specific work; modern physics to photon energy and conservation laws. Draw a free-body or ray diagram in words, list knowns with units, and only then write equations.
Worked example
A 2 kg block starts from rest and slides 5 m vertically down a frictionless 30° incline. Find its speed at the bottom. Energy is simplest: mgh = ½mv², so v = √(2gh) = √(2 × 9.8 × 5) ≈ 9.9 m s⁻¹. The mass cancels. If you use kinematics instead, the acceleration along the incline is g sin30° = 4.9 m s⁻² and the distance along the incline is 5/sin30° = 10 m; then v² = 2 × 4.9 × 10 = 98, giving the same result. Note that the vertical drop, not the incline length, sets the energy change.
Traps that cost marks
- Using the horizontal or incline distance in place of vertical height in mgh.
- Sign errors in lens/mirror equations and in thermodynamic work.
- Forgetting internal resistance when a battery drives a current.
- Adding forces that are not on the same body, or omitting the normal force.
- Mixing rms and peak values in AC power.
- Quoting more significant figures than the data support.