I've been doing some past-papers in order to revise for an upcoming test, and I can't seem to get my head around this question:
A student stands in the middle of a balanced plank which sits on rollers on top of a column. There is zero friction between the plank and the top of the column due to the excellent quality of the rollers. If the student walks (glides) to the right, smoothly and without bouncing state and explain, using Newton’s Laws, what happens to the plank, and the balancing of the system on top of the column.
In my mind, as the person moves to the right, their weight acts on the plank and causes there to be an anticlockwise moment about the pivot. This would then lead to the plank sliding off the rollers as it rotates. However, according to the mark-scheme this isn't the case, with their reasoning being:
The plank will remain balanced. As the student walks, by Newton’s 1st Law (or can argue from 2nd) there is no resultant external force acting and so the centre of mass remains in the same place (above the top of the column)
I don't understand how this is the case, as surely the weight of the student is an external force? Can anybody help me out with what I'm missing here?