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8.2Fluids

Pressure — AP PHYSICS 1 Revision Notes & Topical Questions

5 min read 9 parts Lesson 6 of 14 1 quiz

📚 AP PHYSICS 1 Hub / Topical Revision
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Lesson summary

8.2 Pressure — AP PHYSICS 1 notes: Describe the pressure exerted on a surface by a force and the pressure exerted by a fluid.Pressure = the magnitude of the perpendicular force component exerted per unit area, P = F⊥/A.Unit — pascal, 1 Pa = 1 N/m2; also N/cm2.Scalar — pressure ha…

  • Angled force
  • Solid on a surface
  • Particle model
  • Incompressible fluid
  • Static fluid
  • Same depth

Part 1 of 7Definition of pressure

Pressure
Pressure the magnitude of the perpendicular force component exerted per unit area, P = F⊥/A.
Unit
  • Unit pascal, 1 Pa = 1 N/m2; also N/cm2.
Scalar
  • Scalar pressure has magnitude only;
    • at a point in a static fluid it acts equally in all directions and the force on a surface is perpendicular to it.
Angled force
  • Angled force only the perpendicular component produces pressure.
Size
  • 1 Pa is very small; atmospheric pressure is about 1.0 × 105 Pa.

Part 2 of 7Area and pressure

ObjectAreaPressureReason
Drawing pin point, knife blade, needleSmallHighPierces or cuts easily
Snowshoes, tractor tyres, camel feetLargeLowSpread weight, do not sink
High heelVery smallVery highDamages floors
Rule
  • Rule the same force on a smaller area gives more pressure;
    • standing on one foot doubles the pressure on the floor.
Solid on a surface
  • P = mg/A with A the contact area.

Part 3 of 7Pressure in a fluid

Pressure in a fluid
Particle model
  • Particle model pressure arises from collisions of particles with a surface;
    • each collision changes the particle's momentum,
  • so by Newton's third law the surface feels a force.
Incompressible fluid
  • Incompressible fluid its volume and density stay constant whatever the pressure.
Static fluid
  • Static fluid the pressure depends only on depth, not on container shape or horizontal position.
Same depth
  • Same depth points at the same depth in a connected fluid have the same pressure.

Part 4 of 7Absolute and gauge pressure

Absolute pressure
  • P = P0 + ρgh, with P0 the reference pressure at the surface (often atmospheric, about 1.0 × 105 Pa).
Gauge pressure
  • Pgauge = ρgh, the pressure above atmospheric.
Depth
  • Depth h is the depth below the free surface.

Part 5 of 7Pascal's principle and net force

Pascal's principle
  • Pascal's principle a pressure change applied to an enclosed fluid is transmitted undiminished to every part (hydraulic lifts: F1/A1 = F2/A2).
Net force
  • Net force a pressure difference across a surface gives Fnet = ΔP × A.

Part 6 of 7Calculating pressure at depth

1
Identify the fluid's density and the depth h below the free surface.
2
Identify the surface pressure P0 (often atmospheric).
3
Calculate the gauge pressure ρgh.
4
Add P0 if absolute pressure is required.
5
If a force is needed, multiply the pressure (or pressure difference) by the area.

Part 7 of 7More from the specification

More from the specification
Static pressure in incompressible fluids
  • Static pressure in incompressible fluids measuring fluid thrust acting normally per square metre across fluids provides a means to calculate total pressure.
Ideal fluid
  • Incompressible fluid with no viscosity.
Displacement
  • Change in position.
Pressure
  • P = F/A, the force acting perpendicular to a surface per unit area, measured in pascals (1 Pa = 1 N/m2).
    What the symbols mean: p = pressure (Pa = N m−2) · F = force acting at right angles to the surface (N) · A = area (m2)
Pressure at depth
  • Pressure at depth in a fluid at rest, P = ρgh,
  • so pressure increases with depth.
    What the symbols mean: p = pressure due to the liquid (Pa) · ρ = density of the liquid (kg m−3) · g = 9.81 N kg−1 · h = depth (m)
Direction
  • Direction fluid pressure acts in all directions and pushes at right angles to any surface.
Gauge pressure
  • Gauge pressure the pressure above atmospheric pressure, equal to absolute pressure minus atmospheric pressure.

Key terms

TermMeaning
PressureForce per unit area, P = F/A.
PascalUnit of pressure, 1 N/m2.
Absolute pressureTotal pressure, P0 + ρgh.
Gauge pressurePressure above atmospheric, ρgh.
Pascal's principlePressure change transmits undiminished through an enclosed fluid.

Avoid these mistakes

1
Using area in cm2 without converting to m2.
2
Calling pressure a vector.
3
Forgetting to add atmospheric pressure for absolute pressure.
4
Using depth measured from the container bottom.

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