A force that can change the velocity of a body of mass 1kg from 20 m/s to 30 m/s in 2 seconds must be equal to :

Given, 

Initial velocity , u = 20 m/s

Final velocity, v= 30 m/s

Time taken, t = 2 s

Mass of the body, m = 1 kg

Therefore, acceleration, $\mathrm{a}=\frac{\left(\mathrm{v}-\mathrm{u}\right)}{\mathrm{t}}$

$\Rightarrow a= \frac{\left(30-20\right)}{2}$

$\Rightarrow a=\frac{10}{2}$

$\Rightarrow a= 5 m/s^2$

Now, we know, Force (F) formula as: 

$\Rightarrow F=m\times a$

$\Rightarrow F= 1 kg \times 5 m/s^2$

$\Rightarrow F= 5N$

Therefore, the force will be equal to 5 N.