Understanding Newton's Laws of Motion: Solving a Tricky Equation with Blocks

[azizlwl]

At the instant that one-fourth of block 1 remains on block 2, x₂+l=x₁+(l/16).

For days trying to figure out how this equation derived from.
Thank You.

http://img15.imageshack.us/img15/3926/kotak.jpg

 

[tiny-tim]

At the instant that one-fourth of block 1 remains on block 2, x₂+l = x₁+(l/16).

hi azizlwl!

(i don't understand why they've put lengths on both sides of the equation, but …)

x₁ and x₂ are the distances blocks 1 and 2 have moved

since block 1 has to overtake block 2 by (15/16)l, that means x₁ - x₂ = (15/16)l