4.3.5—The upthrust acting on an object in a fluid is due to a difference in
- Syllabus
- 9702–2028–2029
- Objective
- 4.3.5
- Level
- AS
Upthrust comes from the pressure difference between the lower and upper surfaces of an immersed object
Fluid pressure is greater at the lower surface than at the upper surface, giving a resultant upward force called upthrust.
The net force is caused by the pressure gradient, not by a mysterious property of the object. Compare upthrust with weight to predict rise, sink or equilibrium.
A submerged block has larger pressure on its bottom face than its top face, so the vertical pressure forces do not cancel.
Upthrust is not always equal to weight; equality occurs only when the object has no vertical acceleration.
ConceptA-Level CAIE Physics AS