Initial velocity of the truck, u = 0 m/s

Time taken, t = 20 s

Distance covered by the stone, s = 400 m

According to the second equation of motion:

s = ut + 1/ 2 at^{ 2}

We have

400 = 0 × 20 + 1 /2 × × (20)^{ 2 }

⇒ 400 = 200

⇒ = 2 m/s2

Now, Force = mass × acceleration

⇒ F = 7000 × 2 = 14000 N