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Theme A — Space, Time and Motion

A.1.7 Fluid Resistance — IB Physics HL Revision Notes & Topical Questions

5 min read 10 parts Lesson 9 of 63 1 quiz

📚 IB Physics HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

A.1.7 Fluid Resistance — IB PHYSICS SL notes: Describe fluid resistance (drag) and lift, the factors that affect them, and their influence on motion.Fluid resistance = the force exerted by a liquid or gas that opposes the relative motion of a body moving through it.It is also cal…

  • Type of force
  • Opposite to motion
  • Cannot start motion
  • Slow or maintain
  • Energy transfer
  • Directions

Section 1 of 10What fluid resistance is

What fluid resistance is
Fluid resistance
Fluid resistance the force exerted by a liquid or gas that opposes the relative motion of a body moving through it.
  • It is also called viscous drag, or air resistance in air.
Origin
  • Origin the body pushes fluid out of its way and fluid layers rub against its surface.
Direction
  • Direction it acts opposite to the velocity of the body relative to the fluid.
Type of force
  • Type of force it is a frictional force that transfers kinetic energy away from the object to the thermal energy of the surroundings.

Section 2 of 10Properties of frictional forces

Opposite to motion
  • Opposite to motion they always act against the direction of motion.
Cannot start motion
  • Cannot start motion they never speed an object up or set it moving.
Slow or maintain
  • Slow or maintain they slow an object down or keep it at constant speed against a driving force.
Energy transfer
  • Energy transfer they always transfer energy away from the object.

Section 3 of 10Lift

Lift
  • Lift an upward force on an object moving through a fluid, perpendicular to the fluid flow.
  • A wing pushes air downwards and by Newton's third law the air pushes the wing upwards.
Directions
  • Directions drag is opposite to the thrust (motion) and lift is opposite to the weight.

Section 4 of 10Factors affecting drag

Speed
  • Speed drag increases as the speed relative to the fluid increases.
  • At low speed in a viscous fluid it is roughly proportional to speed.
  • In air at higher speeds it grows roughly with the square of the speed.
Cross-sectional area
  • Cross-sectional area a larger frontal area pushes aside more fluid and feels more drag.
Shape
  • Shape streamlined shapes reduce drag and blunt shapes create turbulent wakes that increase it.
Fluid properties
  • Fluid properties a denser or more viscous fluid gives more drag.
Surface roughness
  • Surface roughness rougher surfaces increase the frictional part of drag.

Section 5 of 10Laminar and turbulent flow

1Laminar flow
  • Laminar flow smooth parallel layers at low speed past smooth shapes, with small drag proportional to speed.
2Turbulent flow
  • Turbulent flow swirling eddies at high speed or with blunt shapes, forming a low-pressure wake.
  • The pressure difference between front and back raises drag, roughly as v2.
3Stokes' law
  • Stokes' law it gives the drag precisely on a small sphere moving slowly in a viscous liquid.

Section 6 of 10Effect on motion

Falling object
  • Falling object the resultant force is Fnet = mg − Fdrag, so a = g − Fdrag/m is less than g.
Terminal speed
  • Terminal speed as speed rises drag grows until it balances the weight and the acceleration falls to zero.
Projectiles
  • Projectiles drag reduces both the horizontal and vertical speeds.
Parachute
  • Parachute a larger area increases drag and reduces the terminal speed.

Section 7 of 10Proportional reasoning and design

Quadratic drag
  • Quadratic drag if drag is proportional to v2, doubling the speed multiplies the drag by four.
Power against drag
  • P = Fv then rises as v3, so doubling speed needs about eight times the power.
Reducing drag
  • Reducing drag streamlined bodies of fish, birds, cars and cyclists' helmets lower drag and save energy.
Using drag
  • Using drag parachutes, aircraft air brakes and shuttlecock fins increase resistance on purpose.
Paper cone experiment
  • Paper cone experiment a larger area gives a longer fall time over a fixed distance.
Spacecraft re-entry
  • Spacecraft re-entry drag converts kinetic energy into thermal energy, which a heat shield must dissipate.

Section 8 of 10More from the specification

1Drag dependence across velocity range
  • Drag dependence across velocity range viscous fluid resistance transitions from linear laminar drag to quadratic turbulence across a wide velocity range.
2Fluid Resistance and Terminal Velocity
  • An object falling through a fluid experiences an upward drag force that increases with speed until it balances gravitational weight,
  • resulting in zero net force and maximum terminal velocity.
3Velocity-Time Graph for Resisted Motion
  • On a velocity-time graph of an object falling through a fluid, the gradient represents instantaneous acceleration,
  • which decreases continuously from g to zero as fluid resistance grows.
4Effects of Drag on Projectile Trajectory
  • Fluid resistance causes the horizontal velocity to decay and reduces maximum height, distorting the symmetric parabolic trajectory into an asymmetric path with a steeper downward descent.
5Displacement and Kinematic Vector Equations
  • Displacement measures net positional change from the initial origin;
    • choosing downward motion as positive assigns a negative sign to upward initial velocities and resistive decelerations.
6Average speed
  • Total distance divided by total time.
7Scalar quantity
  • A quantity with magnitude only, such as potential
8Average velocity
  • Displacement divided by time.
9Displacement
  • Change in position.
10Gradient
  • Slope of a graph line.
11Vector
  • Quantity with magnitude and direction.
12Maximum height
  • The greatest height reached, where the vertical velocity is zero.
13Initial velocity
  • Velocity at the start, u.
14Kinematic equations
  • Equations relating displacement, velocity, acceleration and time for constant acceleration.
15Final velocity
  • Velocity at the end, v.
16Velocity-time graph
  • A graph of velocity against time whose gradient is acceleration and area is displacement.

Section 9 of 10Key terms

TermMeaning
Fluid resistanceForce opposing motion through a fluid
DragAnother name for fluid resistance (viscous drag)
LiftUpward force on a body moving through a fluid, perpendicular to the flow
StreamlinedShaped to reduce turbulence and drag
Laminar flowSmooth flow in parallel layers
Turbulent flowIrregular flow with eddies
ViscosityMeasure of a fluid's resistance to flow

Section 10 of 10Exam technique

1
Saying drag is constant — it increases with speed.
2
Thinking a heavier object always falls faster — it depends on the balance of weight and drag.
3
Confusing drag with friction between solids — drag needs a fluid and depends on speed.
4
Forgetting that drag acts opposite to the velocity relative to the fluid — wind changes the relative velocity.
5
Assuming doubling speed doubles drag in air — it roughly quadruples it.

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