A large cargo ship floats on the sea. The ship has a flat-bottomed hull with a total area in contact with the water of 4000 m². The same ship, when fully loaded with cargo, sinks deeper into the water. Explain why the fully loaded ship exerts a greater pressure on the water beneath its hull than the empty ship does.

Cambridge International IGCSE Physics (0625) — 1.8 Pressure · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Pressure is defined as the force per unit area acting on a surface. When an object rests on or in a fluid, it exerts a downward force equal to its weight over the area of contact.

Model answer (4 marks)

The loaded ship has a larger weight, so the downward force on the hull increases.
The hull’s contact area with the water stays essentially the same (4000 m²) or does not increase enough to offset the added weight.
Pressure is force divided by area. With a larger force and the same area, the pressure on the water increases.
Thus the fully loaded ship exerts a greater pressure on the water beneath its hull than the empty ship.

Examiner tips

  • Use the definition of pressure (P=F/A) to link force and area.
  • Show the weight increase first, then note the area is unchanged.
  • Explain the consequence for pressure clearly.

Mark scheme (4 marks)

  1. Loading the ship increases its total weight (downward force)
  2. The area of the hull in contact with the water remains the same (or does not increase sufficiently to compensate)
  3. Pressure = force ÷ area, so greater force with the same area gives greater pressure
  4. Therefore the pressure on the water beneath the hull is greater for the loaded ship

Key terms in this question

pressure · area

Related

More Pressure questions

▶ Try answering this question with AI marking (free) →