Solids, Liquids and Gases
4 marks · EasyA block of soap rests on a bathroom shelf. The mass of the soap is 200 g and its base measures 8.0 cm by 5.0 cm.
Calculate the area of the base of the soap in m².
Calculate the pressure the soap exerts on the shelf, stating the equation used.
Give the correct SI unit for pressure.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) 0.004 m²
(b) Pressure = 0.2 kg × 10 N/kg / 0.004 m² = 500 Pa · must show p = F / A
(c) pascal
Common mistakes: not converting cm² to m²; using mass instead of weight (force)
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Converts cm to m, finds area as length × width.
M2 (b) · 1 mark · Writes and/or uses correct equation for pressure (p = F/A or p = W/A).
A1 (b) · 1 mark · Substitutes values correctly, calculates pressure as 500 Pa (accept values 495–505 Pa). (follow-through)
B1 (c) · 1 mark · Gives correct SI unit (pascal or Pa).
Worked solution
(a) Convert length and width to metres: 8.0 cm = 0.08 m, 5.0 cm = 0.05 m
(b) Pressure equation:
Mass = 200 g = 0.2 kg. Weight (force) = N
(c) The SI unit for pressure is the pascal (Pa).