Pressure in Liquids and Gases
4 marks · NormalA deep-sea diver is submerged at a certain depth in a stationary sea. The pressure at the diver's position is measured in three directions: upwards, downwards, and horizontally.
Explain why the pressure at a point in a stationary liquid is the same in each of these directions.
If the diver is not moving and the water around them is stationary, what would you expect a pressure gauge to read if pointed in any of these three directions at the same point? Explain your answer.
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(a) Pressure at a point in a stationary liquid is the same in all directions because molecules are constantly moving and colliding, transmitting force equally at all points. · must show Reference to stationary liquid/gas, Explanation involving molecular motion/collisions or balance of forces
(b) The pressure gauge will read the same value in all three directions because pressure acts equally in all directions at a point in the liquid. · must show Pressure is the same in all directions, Reference to gauge reading being equal
Common mistakes: Thinks pressure is greater in one direction; Thinks pressure changes with direction in a still liquid
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Marking points
M1 (a) · 1 mark · States pressure acts equally in all directions at a point in stationary liquid.
A1 (a) · 1 mark · Gives clear explanation referencing molecular motion/collisions or balance of forces.
B1 (b) · 1 mark · States gauge will show same reading in all directions.
B2 (b) · 1 mark · Explains because pressure acts equally in all directions.
Worked solution
a) Pressure at a point in a stationary liquid is the same in all directions because the liquid molecules move equally in all directions and transmit force equally. There are no currents, so no direction is different.
b) The pressure gauge would read the same value whether pointed up, down, or sideways at the diver's position, because pressure acts equally in every direction at that location in the stationary liquid.