Physics - Mechanics, Electricity and Energy Reasoning
The physics paper at the ENP Alger access contest concentrates on mechanics and electricity, with a smaller block of oscillations, thermal physics or radioactivity taken from the final-year scientific syllabus. Questions are usually quantitative and multi-step: a system is described in words or a short text, and you must choose the laws, set up equations and carry units consistently to the end.
Recurring content
- Mechanics: kinematics in one and two dimensions, Newton's second law, friction, work–energy theorem, conservation of momentum, projectile motion.
- Electricity: Coulomb's law, field and potential, capacitors and RC transients, force on a moving charge in a magnetic field.
- Oscillations: harmonic motion, period of a pendulum and of a spring–mass system.
- Thermal and nuclear: calorimetry, first law of thermodynamics, radioactive decay law N = N₀e^(−λt).
Method that earns marks
- Draw the free-body diagram before writing any equation, then fix the axes once and keep them.
- Write the energy relation with non-conservative forces explicitly: ΔK = W_conservative + W_non-conservative.
- Keep symbols until the final line, then substitute SI values; g is usually taken as 10 m/s² in contest papers unless stated otherwise.
Worked example
A 1000 kg car accelerates from rest to 20 m/s in 10 s against a constant friction force of 500 N. What is the average engine power? Uniform acceleration a = Δv/Δt = 2 m/s², so the distance is d = ½at² = 100 m. The kinetic energy gained is ΔK = ½mv² = ½ × 1000 × 400 = 2.0×10⁵ J. Work against friction is 500 × 100 = 5.0×10⁴ J. The engine therefore supplies 2.5×10⁵ J in 10 s, giving P = 2.5×10⁴ W = 25 kW. The step candidates lose is the friction term, which is 20 % of the total here.
Traps that cost marks
- Mixing grams with newtons, or centimetres with metres, inside the same formula.
- Forgetting that in an RC circuit the capacitor voltage cannot jump while the current can.
- Misusing the time constant: at t = τ the charge reaches about 63 % of its final value, and roughly 99 % only at 5τ.
- Sign errors in projectiles: at the top v_y = 0 but the acceleration is still −g.
- Reporting more significant figures than the data justify.