The driving pulley and the driven pulley in an open-flat-belt transmission each have a diameter of 160 mm. Calculate the preload needed in the belt if a power of 7 kW should be transmitted at 1000 rpm by using only one-half the wrap angle on each pulley. The coefficient of friction is 0.25. the density of the belt material is 1500 kg/m^3, and the allowable belt preload is 5 MPa.