Moments and pressure

Revision notes

Pressure is force normal to a surface divided by area, p = \(\dfrac{F}{A}\). In a fluid, pressure acts in all directions and generally increases with depth. A pressure difference across a submerged object can produce an upward resultant force called upthrust. A stationary floating object has balanced vertical forces, so upthrust equals weight.

Pressure on surfaces and in fluids

Pressure is force per unit area, p = \(\dfrac{F}{A}\). A smaller contact area gives a larger pressure for the same normal force. Convert area to square metres before calculating pressure in pascals.

In fluids, pressure acts in all directions and generally increases with depth. A submerged object can experience a larger pressure on its lower surface than on its upper surface, producing an upward resultant force called upthrust. A stationary floating object has upthrust equal to its weight.

Exam technique

For floating questions, state that upthrust equals weight when the object is stationary, and explain upthrust using the greater pressure on the lower surface.