A block takes twice as much time to slide down a 45° rough inclined plane as it takes to slide down a similar smooth plane. The coefficient of friction is:

By using 2nd equation of motion s = ut + (½) at2For smooth inclined plane :s=0+12(gsin⁡θ)t12For sough inclined plane :s=0+12(gsin⁡θ−μgcos⁡θ)t22But t2=2t1 so μ=34