Summary

• Pressure is the ratio of normal force to area.
  • Although both force and area are vectors, pressure is a scalar quantity and has no direction.
  • Pressure is a way to describe force in a region of a continuous system.

  P =	F
  	A

• Units
  • The SI unit of pressure is the pascal [Pa = N/m² = kg/ms²].
  • One hundred thousand pascals are sometimes called a bar [100,000 Pa = 1 bar].
  • The unit atmosphere is 101,325 Pa by definition.
• Absolute vs. Gauge
  • The pressure of a region, as defined above, is its absolute pressure (P).
  • The pressure difference between a system and its environment (P₀) is known as gauge pressure (P_g).

  P = P0 + Pg

• Pressure in a fluid (P) at rest …
  • is equal to the weight of a column of fluid divided by the area on which it rests, so that it …
    • increases uniformly with depth (h)
    • is directly proportional to the density of the fluid (ρ)
    • depends on the surface pressure (P₀)

    P = P0 + ρgh

  • acts equally in all directions and therefore …
    • exerts a net force perpendicular to any surface that it contacts
• Pascal's principle: Pressure changes applied to the surface of an enclosed fluid are transmitted evenly throughout the fluid.
  • Hydraulics increase force, but

    P1 =	F1	=	F2	= P2
    	A1		A2

  • decrease distance (since energy is conserved)

    W1 = F1s1 = F2s2 = W2