The pressure in fluids causes a force normal (at right angles) to any…

The idea

Pressure is not a force with a direction of its own. The particles of a fluid move randomly and strike whatever surface they meet, and averaged over many collisions the push is always at right angles to that surface, whichever way the surface faces. So the water in a tank pushes outwards on the sides as well as down on the base, and the air pushes up on the underside of a table as hard as it pushes down on the top. The size of the force follows from the area, so the same pressure produces a much larger force on a large window than on a small one. Where the pressures on the two faces of a surface differ, the resultant force is normal too, and it points from the high pressure side towards the low pressure side.

What earns the marks

• The force acting at right angles, or normal, to the surface.
• The force arising from fluid particles colliding with the surface.
• Stating that this holds whatever the orientation of the surface.
• A resultant force found from the difference in pressure multiplied by the area.

Common mistakes and how to put them right

• Drawing the force from a fluid as acting downwards only.

• Assuming a fluid presses on the base of a container but not on its sides.

• Using an area in square centimetres without converting it to square metres.

• Calculating a resultant from the larger pressure alone rather than from the difference.

Exam tips

• Draw the arrow perpendicular to the surface, not vertically down the page.
• For a resultant force, subtract the two pressures first and then multiply by the area once.