When I calculate the weight of the 81.6 kg, the number I use for gravity is 9.807 m/s². That gives a weight of 800.25 N, so I think that's where the question got the crazy number of 81.6 kg ... whoever wrote the problem wants the hay to weigh 800 N, and that's what I'll use for the weight.
The forces on the bale of hay are gravity: 800N downward, and the guy on the truck with the pitchfork pulling upward on it with 850 N. The net force on the bale is (850 - 800) = 50 N upward.
Use Newton's second law of motion: (Net force) = (mass) x (acceleration)
Divide each side by 'mass' :
Acceleration = (net force)/(mass)
On the hay wagon,
Acceleration = (50 N upward) / (81.6 kg) = <em>0.613 m/s² upward</em>
