A rectangular billboard of height 2.5 m2.5\text{ m}2.5 m and length 8.0 m8.0\text{ m}8.0 m is mounted vertically on a flat field.
Wind of speed 15 m s−115\text{ m s}^{-1}15 m s−1 blows at right angles to the front face of the billboard for 2.0 s2.0\text{ s}2.0 s. The density of air is 1.2 kg m−31.2\text{ kg m}^{-3}1.2 kg m−3.
Show that the mass of air which hits the billboard in this time is 720 kg720\text{ kg}720 kg.
All the air incident on the billboard is deflected at 90∘90^\circ90∘ to its original direction, parallel to the surface of the billboard. Calculate the magnitude of the force FFF exerted by the wind on the billboard.
