Consider the situation shown in figure. Suppose a small electric field E exists in the space in the vertically upward direction and the upper block carries a positive charge Q on its top surface. The friction coefficient between the two blocks is `mu` but teh floor is smooth. what maximum horizontal force F can be applied without disturbing the equilibrium?

Explanation

`R_1+QE=mg=0`
`rarr R_1=mg-QE`…………i

`F-T=muR_1`
From equation i.

`rarr T-mumg+muQE`
=0 …………ii
`T-muR_1=0`
`rarr T=muR_1`
=mu(mg-QE)`……iii
From equation ii
`F-mumg+muQE-mumg+muQE=0`
`rarr F-2mumg+2muQE=0`
`rarr F=2mumg-2muQE`
`rarr F=2mu(mg-QE)`
Maximum horizontal force that can be applied is `2mu(mg-QE)`.