A 20-kg block is attached to a spring of constant k = 8 kN/m and can move without friction...

A 20-kg block is attached to a spring of constant k = 8 kN/m and can move without friction in a vertical slot as shown. The block is acted upon by a periodic force of magnitude P = Pm sin ωf t, where Pm = 10 N. Knowing that the amplitude of the motion is 3 mm, determine the value of ωf .

Fig. P19.100