A truck starts from rest and rolls down a hill with constant acceleration. It travels a distance of $400\mathit{ m}$ in $20s$. Find its acceleration and the force acting on it if its mass is $7$ tonnes.

Acceleration and Force acting on it, if its mass is $7$ tonnes is $2\mathit{ m}/s^2$ and $1.4\times 10^4\mathit{ N}$.
Given, that the truck starts from rest, its initial velocity, $\mathit{u }= 0$.
Distance, $\mathit{s }= 400\mathit{ m}$
Time,$\mathit{ t }= 20\mathit{ s}$
Mass of the truck, $\mathit{m }= 7\mathit{ tn}$
⇒ $m=7\times 10^3\mathit{ kg}$
From Newton’s second law of motion, 
$s=\mathit{ut}+ \frac{1}{2}a{t}^2$
⇒ $400=0\times 20+ \frac{1}{2} \times \mathit{a }\times {\left(20\right)}^2$
⇒ $400=200\mathit{ a}$  ⇒ $a=2\mathit{ m}{s}^{-2}$
The force acting on the truck is,
$F=\mathit{ma}$
    $=7\times {10}^3\times 2$
    $=14000\mathit{ N}$
    $= 1.4\times {10}^4\mathit{ N}$
∴ The acceleration of the truck is $2\mathit{ m}/s^2$ and the force acting on it is $1.4\times 10^{4 }N.$