Fluids exert pressure in every direction
Definition
Fluid
A substance that can flow, so liquids and gases are fluids.
- Liquids and gases are fluids because their particles can move past one another and the substance can flow.
- Within a fluid, moving particles and the weight of fluid above create pressure.
- At a point in an open fluid, the total pressure includes both the fluid's pressure and the atmospheric pressure acting on its surface.
Pressure produces a normal force
Definition
Pressure
The normal force acting per unit area of a surface.
- Fluid pressure exerts a force normal to a surface, meaning perpendicular to that surface.
- The force acts on every surface in contact with the fluid, including horizontal, vertical and curved surfaces.
- Because the force is normal locally, its direction changes around a curved object.
Example
Forces on a submerged object
- Water pushes down on the top, sideways on the vertical faces and up on the bottom of a submerged block.
- Each force acts at right angles to the surface where it is applied.
Open and sealed fluids differ
- For an open container, atmospheric pressure is transmitted through the liquid as part of the total pressure.
- A pressure gauge may show gauge pressure, which is the amount above atmospheric pressure, rather than total pressure.
Common Mistake
Do not draw fluid-pressure forces parallel to a surface; each force must be perpendicular to the surface.
Self review
- What is a fluid?
- What contributes to the total pressure in an open liquid?
- In what direction does pressure exert force on a surface?
- How do pressure-force directions vary around a curved object?
