Stack of Blocks:

This is a problem about a child pushing a stack of two blocks along a horizontal floor. The masses of the blocks, and the coefficients of friction between the two blocks and between the lower block and the floor will be given. In order to do the pushing, the child will only be touching one of the two blocks.

The mass of the upper block in the stack is 0.680 kg. The mass of the lower block in the stack is 1.530 kg . The coefficients of friction between the two blocks are: static 0.930, and kinetic 0.640. The child's mother, who likes to encourage his experiments, has oiled a small strip of the horizontal floor so that it is very slick; the coefficient of kinetic friction between the oiled section of floor and the lower block is only 0.090 and the coefficient of static friction is insignificantly different.

Before the pushing starts, here is a question about the vertical forces acting on the two blocks.

The Upper Block Slips Off the Lower Block
Now the child starts to push with a horizontal force of 8.586 N to the North. The upper block immediately begins to slip off of the lower block. The next set of questions are about the time interval during which the upper block is slipping off of the lower block, but while the two blocks are still in contact.

What will be the acceleration of the upper block during this interval? Give the horizontal component, with North positive.