∴Rt⊿AOB proves AB=5 by Pythagorean theorem.

That is sin∠OAB=3/5.

sin∠OAB=OC/OA。

∴OC=(3/5)OA

It is easy to know that when O'c'= 1, the circle is tangent to the straight line.

O'A=5/3, so OO'=4-5/3=7/3.

At this time, only 7/3÷ 1=7/3 seconds passed.