Overview
This lecture covers how to calculate the pressure exerted by a column of liquid and explains the factors influencing whether an object floats or sinks.
Calculating Pressure in Liquids
- Pressure in a liquid increases with depth due to the weight of the liquid above.
- The equation for liquid pressure is: Pressure (Pa) = height (m) × density (kg/m³) × gravitational field strength (N/kg).
- In sample calculation, an 8 m deep freshwater lake with density 1,000 kg/m³ and gravity 9.8 N/kg gives 78,400 Pa at the bottom.
- Pressure is higher at the bottom of a container than at the top because of greater depth.
Factors Affecting Pressure and Density
- As liquid depth increases, the pressure exerted increases.
- Higher liquid density also results in higher pressure at the same depth.
Floating and Sinking: Upthrust
- The bottom of a submerged object experiences more pressure than the top, creating an upward force called upthrust.
- Upthrust is the upward force acting on an object due to liquid pressure differences at different depths.
- An object floats if upthrust equals its weight; it sinks if upthrust is less than its weight.
Displacement and Floating Behavior
- Upthrust equals the weight of the water displaced by the object.
- Objects less dense than water need to displace a small volume of water to float high.
- Objects with the same density as water float at the surface after displacing their own volume of water.
- Objects more dense than water cannot displace enough water to balance their weight and thus sink.
Key Terms & Definitions
- Pressure — Force exerted per unit area, measured in Pascals (Pa).
- Density — Mass per unit volume, measured in kg/m³.
- Upthrust — Resultant upward force on a submerged object, caused by pressure difference between top and bottom.
- Displacement — The process of an object pushing water out of the way as it is submerged, equal to the volume of the object submerged.
Action Items / Next Steps
- Practice more questions on liquid pressure from the revision workbook.
- Review the pressure formula and how to apply it with different depths and densities.