From Mechanics

As the intercept is 0 and the gradient is \displaystyle \frac{8000}{20}=400 , using the equation for a straight line, the force at a time \displaystyle t is given by:

\displaystyle F=400t+0=400t

As the mass is 1000 kg, using Newtons's second law, the acceleration is given by:

\displaystyle a=\frac{F}{1000}=\frac{400t}{1000}=\frac{2t}{5}