A horizontal turntable is initially at rest. It starts to rotate in the horizontal plane with constant angular acceleration $3\,\text{rad/s}^2$. A block of mass $1\,\text{kg}$ is placed $1\,\text{m}$ away from the axis of rotation on the turntable. A horizontal force $F = 5t\,\text{N}$ (where $t$ is in seconds) acts on the block always towards the center of rotation. The block begins to slip at $t = 1\,\text{s}$. The coefficient of friction between the block and the turntable is