A paint-spraying device is made from a box whose faces are arranged horizontally and vertically. Small openings are present in one of the vertical faces. Paint is supplied into the box at pressure through a vertical tube, then leaves the openings as narrow jets travelling horizontally. The paint leaves at a speed of $2.5\,\text{m s}^{-1}$ through $400$ holes, each with area $0.4\,\text{mm}^2$. The density of the paint is $900\,\text{kg m}^{-3}$. Calculate the horizontal force needed to keep the device at rest while the paint is being sprayed.
- A$0.36\,\text{N}$
- B$0.90\,\text{N}$
- C$2.3\,\text{N}$
- D$900\,\text{N}$