a mechanic problem

圖片參考：http://imgcld.yimg.com/8/n/HA00028060/o/700911080018013873407200.jpg

Let x be the distance travel of m1,and d be the distance travel by m2.
By law of conservation of energy:

圖片參考：http://imgcld.yimg.com/8/n/HA00028060/o/700911080018013873407211.jpg

Now,consider the force.

圖片參考：http://imgcld.yimg.com/8/n/HA00028060/o/700911080018013873407212.jpg

***This is a physical constringe.When m1 move up x units.The lenth of the string between the pulley and m2 should be a+x/2
The diagram should be like this:

圖片參考：http://imgcld.yimg.com/8/n/HA00028060/o/700911080018013873407223.jpg

Finally,let's make a simple calculation.

圖片參考：http://imgcld.yimg.com/8/n/HA00028060/o/700911080018013873407224.jpg

Change the subject be d and the answer will show obviously.