With an emergency stop of a train moving at a speed of 72 km / h, the braking distance was 100 m.

With an emergency stop of a train moving at a speed of 72 km / h, the braking distance was 100 m. What is the coefficient of friction between the train wheels and the rails?

Initial data: V (train speed before the emergency stop) 72 km / h; S (the length of the braking distance of the train) = 100 m.

Constants: g (acceleration due to gravity) = 10 m / s ^ 2.

SI system: 72 km / h = 20 m / s.

1) Calculate the acceleration of the train when braking (a <0):

S = (V ^ 2 – V0 ^ 2) / 2a = -V0 ^ 2 / 2a (V0 = 0 since the train will stop).

a = -V0 ^ 2 / 2S = -20 ^ 2 / (2 * 100) = -2 m / s ^ 2.

2) We calculate the coefficient of friction between the wheels of the train and the rails:

m * a = Ftr. = μ * m * g; a = μ * g.

μ = a / g = 2/10 = 0.2.