Answer:
 F = 1.68 N
Explanation:
Let's solve this exercise in parts.
Let's use the concept of conservation of the mechanical nerve
initial
  Em₀ = 500 J
The energy is totally kinetic
   Em₀ = K = ½ m v₀²
   v₀ = [tex]\sqrt{\frac{2 Em_{o} }{m} }[/tex]
   v₀ = √ (2 500/32)
   v₀ = 5.59 m / s
now with kinematics we can find a space
   v² = v₀² - 2 a x
the negative sign is because the body is stopping
    a =[tex]( \frac{v_{o}^{2} - v^{2} }{2x} )[/tex] Â
let's calculate
    a = (5.59² - 5.1²) / 2 50 Â
    a = 0.0524 m / s²
Finally let's use Newton's second law
   F = ma
   F = 32 0.0524
   F = 1.68 N