P11 Forces and Pressure

P11 Forces and Pressure

1. P11.1 Preasure and surfaces

   • Cross-sectional area and force have an inverse proportionality on the presure acting on a container.
   • P = F/A is the pressure acting on a surface.
   • Pressure can be caused by the action of fluids (free flowing substance, is not restricted to a liquid).
   • Increasing the force applied on a surface increases the pressure acting upon it. Conversely, increasing the cross-sectional area of a surface decreases the force applied on it.
   • F = PA, the force acting on a surface; A = F/P the cross sectioanl area of the surface being acted upon with pressure.
   • Use metric units (systeme internationale [SI] units) in the exam.
   • Pressure can be used as a force multiplier for hydraulic models, or in a variety of contexts. I understand that these may come up on the exam.
   • In the exam I will ensure to use the correct (that is, the units required by the question) SI units and convert where appropriate.
   • If a data sample is given in the form of a table of values, calculate the mean progression to extrapolate the appropriate values and compute the area/pressure/force.

2. P11.2 Pressure in a liquid at rest

   • As an object travels deeper into a fluid, the volume (and by extension mass) increases above the given object. Hence, the pressure upon said object increases with depth due to the increase of the aforementioned factors.
   • P (rho) = hpg, where rho = pressure, h = height, p = density and g = gravitational field strength.
   • A manufacturer may increase/decrease the cross-sectional area of a piston or other such surface to decrease/increase the pressure respectively. Likewise they may use a fluid with a higher density to producev a higher force multiplier.
   • P (rho) = hpg, h = rho/pg, p (density) = rho/hg, g = rho/hp.
   • A dam provides renewable energy from tidal forces and generates hydroelectric power from the weight acting on a body of water. The flow of the water exerts pressure onto a turbine, when the tide is low the water from the estuary is expelled into the adjacent body of water to spin a turbine and therefore generate electricity.