Why Did Ignoring the Ground Reaction Force Yield the Correct Torque Calculation?

[Asad Raza]

Homework Statement

A uniform cylinder has mass m kilograms and radius 5 metres. Calculate the magnitude of the horizontal force Fthat needs to be applied on its axis to start it rolling up a step of height 2 metres, as shown in the diagram

Homework Equations

Anticlockwise torque=Clockwise torque at point of contact of cylinder and block

The Attempt at a Solution

4mg=3F gave me the correct value of F needed to start rolling. But I disregarded the reaction force from the ground and still got the correct answer. How is that possible that disregarding the moment by the reaction on the cylinder from the flat ground had no effect on the answer. Picture of the question has been uploaded too.

 

[haruspex]

How is that possible that disregarding the moment by the reaction on the cylinder from the flat ground had no effect on the answer.

Imagine increasing F slowly until the cylinder just starts to lift. What would be the normal force from the ground then?

 

[Asad Raza]

Imagine increasing F slowly until the cylinder just starts to lift. What would be the normal force from the ground then?

Ooops! Got it :')