The hollow tube is pivoted about a horizontal axis through point O and is made to rotate in the vertical plane with a constant counterclockwise angular velocity θ˙ = 3 rad /sec. If a 0.2-lb particle is sliding in the tube toward O with a velocity of 6 ft/sec relative to the tube when the position θ = 30° is passed, calculate the magnitude N of the normal force exerted by the wall of the tube on the particle at this instant.