Resoluçao:

a = ?                                                             m = 800 + 60 = 860 kg

V = 72km/h = 20m/s                                 F = m.a

R = 20 m                                                     F = 860. 20 = 17200 N

a = V²/R                                                       F = 860 . 10 = 8600 N

a = 20²/20                                                   N = 17200 + 8600 = 25800 N

a = 20 m/s²

Resposta: 25 800 N