Question

superman must stop a 120-km/h train in 150 m to keep it from hitting a stalled car on the tracks. If the trains mass is 3.6 105 kg, how much force must he exert? Compare to the weight of the train (give as %). How much force does the train exert on superman?

Answer 1

Initial velocity: v i = 120 km/h : 3.6 = 33 m/ s

Final velocity : v f = 0 m/s

Displacement : d = 150 m

m = 360000 kg

v f ²= v i ²+ 2 a d

a = - v i ²/ 2 d

a = - (33 m/s)² / 300 m = - 3.63 m/s²

F = m a = 360000 kg * ( - 3.63 m/s² ) = - 1306800 N ≈

≈ - 1.3 * 10 to the power of 6 N

Weight of the train:

W = m g = 360000 * 9.81 = 3531600 N.

Force is 36.82% of the weight.