∴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.