Engine mass M = 8000 KG

Mass of 1 waggon m = 2000 kg

Mass of 5 waggon m = 2000 x 5 kg = 10000 kg

Force exerted by engine F = 40000N

Friction force f = 5000 N

(i) net accelerating force f' = F -f

f' = 40000N - 5000N

f' = 35000N

(ii) Force = total mass x acceleration

f' = (M + m) x a

35000N = (8000 + 10000) kg x a

35000N = 18000 kg x a

35000N/18000kg = a

a = 1.94 ms^-2

(iii) Only on waggon 1, the external force will be applied. The net force on the remaining 4 waggons is equal to the force applied by waggon 1 on 2.

Assume the acceleration on waggons = a'.

f' = m x a'

35000N = 10000 kg x a'

a' = 35000N / 10000 kg 

a' = 3.5 ms^-2

Mass of 4 waggons m'= 4 x 2000 kg

m'= 8000 kg

∴ Net force on last 4 waggons F' = 8000×3.5 

=28000 N

force applied by waggon 1 on 2 is 28000 N