Answer:
v = 6.32 [m/s]
Step-by-step explanation:
We know that work in physics is defined as the product of force by distance, in this way we have:

where:
F = force [N]
W = work [J]
d = distance = 4 [m]
![800=F*4\\F=200[N]](https://img.qammunity.org/2022/formulas/physics/high-school/d6jzd7sgk82s9973mdjrccn77lch2zblob.png)
Now we know that by means of Newton's second law which tells us that the total force on a body is equal to the product of mass by acceleration, we can determine the acceleration that occurs.

where:
F = force = 200 [N]
m = mass = 40 [kg]
a = acceleration [m/s²]
![a=F/m\\a=200/40\\a=5[m/s^(2) ]](https://img.qammunity.org/2022/formulas/physics/high-school/sjvtn8ne2xv3h24yljyd4jmvglkn1tkxwk.png)
Now using this equation of kinematics we can determine the final velocity.

where:
vf = final velocity [m/s]
vo = initial velocity = 0 (initially at rest)
a = acceleration = 5 [m/s]
x = distance = 4 [m]
![v_(f)^(2)=0+(2*5*4)\\v_(f)=√(40)\\v_(f)=6.32[m/s]](https://img.qammunity.org/2022/formulas/physics/high-school/9fzr30zoziqg8z3z32vxhu1zt27dorwxm7.png)