1. The problem statement, all variables and given/known data Estimate (i) the mass of the brakes of a train needed to stop the brake block temperature reaching melting point when the brakes are applied (ii) the number of photons emitted per second by a light bulb An order of magnitude estimate is sufficient. 2. Relevant equations 3. The attempt at a solution For i), I assumed that all of the kinetic energy of the train is converted to thermal kinetic energy of the material. The thermal energy is on the order of kT. I assumed that the speed of the train is 30 m/s and that the mass of the train is 2000 kg. So, kT = 0.5mv2 This gives T = 3.2*1030 K. This is clearly absurd!!!!! For ii), power of a typical light bulb = I2R = (50 mA)2(1.5 ohms) = 380 mW. And energy of one light photon = hc/lamba = 2.9*10-17 J. (assuming lamba = 700 nm). So, number of photons = power of light bulb / energy of one photon = 1.3*1012 s-1, which is a sensible number. I'd be grateful to you to anyone who suggests a much better way.