please explain the "why" as well so I can understand fully. Thank you.

Q16. A brave but inadequate rugby player is being pushed backward by and opposing player who is exerting a force of 800 N on him. The mass of the losing player plus equipment is 90.0kg, and he is accelerating at 1.20 m/s² backward. A) What is the force of friction between the losing player's feet and the grass? B) What force does the winning player exert on the ground to move forward if his mass plus equipment is 110kg? C) Draw a sketch of the situation showing the system of interest used to solve each part.