Question 5
A reservoir is 1.2 m deep. What force does the water produce on an area of 200 cm2 of its bed? (density of water 1000 kg m−3; g = 10 m s−2)
The right answer is the one marked above.
240 N. pressure = 1.2 × 1000 × 10 = 12 000 Pa. 200 cm2 = 0.02 m2. So force = 12 000 × 0.02 = 240 N.
12 000 N is just the pressure — multiplying by the area has been forgotten. 2400 N comes from converting 200 cm2 into 0.2 m2 by mistake; remember that 1 m2 = 10 000 cm2.