Continuity Equation and Flow Rate

💧 Volume Flow Rate: $Q = Av$

Part 1 of 7 — Fluids: Continuity

Now we move from STATIC fluids to MOVING fluids. The first key concept: how fast fluid passes through a region. We measure this with volume flow rate.

In this lesson you will learn:

• The definition $Q = A\,v$
• Units $(m^{3}/s)$ and conversions to L/min, gal/min
• How $Q$ depends on cross-section and speed
• Why $Q$ matters before we get to continuity

Definition

$Q = A\, v$

• $Q$: volume flow rate $(m^{3}/s)$
• $A$: cross-sectional area $(m^{2})$
• $v$: average fluid speed perpendicular to $A$ (m/s)

Why It Works

In time $\Delta t$, fluid moving at speed $v$ travels a distance $v \Delta t$. The volume that crosses the area is:

$\Delta V = A (v \Delta t)$

So:

$Q = \frac{\Delta V}{\Delta t} = A v$

Useful Conversions

Reality Anchors

• A garden hose: ~$10^{-4}$ $m^{3}/s$ (0.1 L/s)
• A bathroom faucet: ~$10^{-4}$ – $10^{-3}$ $m^{3}/s$
• A river the size of the Mississippi: ~$10^{4}$ $m^{3}/s$

Volume Flow Rate Concepts 🎯

Flow Rate Calculations 🧮

1. Pipe area $A = 0.020$ $m^{2}$, fluid speed $v = 3.0$ m/s. $Q$ $(m^{3}/s)$?

2. River area $A = 50$ $m^{2}$, speed 1.2 m/s. $Q$ $(m^{3}/s)$?

3. A faucet delivers 2.0 L/s. Convert to $m^{3}/s$.

Flow Rate Reasoning 🔍

Exit Quiz — Volume Flow Rate ✅

🔁 Continuity Equation: $A_1 v_1 = A_2 v_2$

Part 2 of 7 — Fluids: Continuity

For an incompressible fluid in a closed pipe, no fluid is created or destroyed: what flows in must flow out. This conservation principle is the continuity equation — perhaps the most-used relation in AP fluids.

In this lesson you will learn:

• The statement of continuity for incompressible flow
• The equation $A_1 v_1 = A_2 v_2$
• Why incompressibility is essential
• Common AP situations (faucet streams, pipe constrictions)

Continuity Equation

For an incompressible fluid in a pipe with no leaks/sources:

$\boxed{Q_1 = Q_2 \quad \Rightarrow \quad A_1 v_1 = A_2 v_2}$

The volume flow rate is the same at every cross-section.

Why "Incompressible"?

Liquids (water, oil, blood) compress so little that we treat them as incompressible. For gases at low speeds (Mach < 0.3), this is also approximately true.

Implication: Speed Up Through Narrows

If $A_2 < A_1$, then $v_2 > v_1$. Fluid speeds up in narrower pipes.

$\frac{v_2}{v_1} = \frac{A_1}{A_2}$

Famous Example: Falling Stream From a Faucet

Water falling from a faucet accelerates due to gravity. Continuity then forces the cross-section to shrink as the stream falls. That's why thin streams of water taper toward the bottom.

Common Pitfall

Continuity compares the same pipe at two places; it says nothing about how fast fluid enters a different, unconnected pipe. The same idea written in kilograms per second is the mass flow rate $\rho A v$ — covered in Part 5.

Continuity Concepts 🎯

Continuity Calculations 🧮

1. Water flows at 4 m/s in a pipe of area $0.020$ $m^{2}$. Speed in a narrower section of area $0.0050$ $m^{2}$ (m/s)?

2. A pipe of radius 5 cm carries water at 1 m/s. Pipe narrows to radius 2 cm. New speed (m/s)?

3. A river 80 m wide, 4 m deep, flows at 2.5 m/s. It enters a 20 m wide, 2 m deep gorge. Speed in the gorge (m/s)?

Continuity Reasoning 🔍

Exit Quiz — Continuity ✅

🔻 Pipe Narrowing & Speed

Part 3 of 7 — Fluids: Continuity

When fluid is forced through a smaller cross-section, it speeds up — sometimes dramatically. Engineers use this in nozzles, fire hoses, and Venturi tubes. AP problems often combine narrowing with depth changes.

In this lesson you will learn:

• Speed scaling with area ratios for circular pipes
• Why a fire-hose nozzle creates such a fast jet
• Combining radius changes with continuity
• Common units pitfalls (cm vs m)

Speed Scaling

For incompressible flow:

$\frac{v_2}{v_1} = \frac{A_1}{A_2}$

For circular cross-sections ($A = \pi r^2$):

$\frac{v_2}{v_1} = \left(\frac{r_1}{r_2}\right)^2$

So halving the radius quadruples the speed.

Example: Fire Hose

A fire hose with internal radius 2.5 cm carries water at 3 m/s. The nozzle constricts to radius 0.5 cm:

$v_{nozzle} = 3 \times \left(\frac{2.5}{0.5}\right)^2 = 3 \times 25 = 75 \text{ m/s}$

A 25× speed boost from a 5× radius reduction. (We've ignored Bernoulli effects on pressure here — Part 5–7.)

Common Pitfalls

1. Diameter vs radius: $A = \pi r^2 = \pi (d/2)^2 = \pi d^2/4$. The ratio $r_1/r_2 = d_1/d_2$ — same.
2. cm vs m: Always convert to consistent units OR use ratios (which cancel units).
3. Single pipe vs branching: This part is single-pipe only. Branching is Part 4.

Pipe Narrowing Concepts 🎯

Pipe Narrowing Calculations 🧮

1. A pipe of radius 0.10 m carries water at 0.5 m/s. Pipe narrows to radius 0.05 m. New speed (m/s)?

2. A garden hose of inner diameter 2.0 cm runs at 1.0 m/s. Nozzle diameter 0.50 cm. Speed at nozzle (m/s)?

3. A pipe with $A_1 = 6.0\times10^{-3}$ $m^{2}$ and $v_1 = 4.0$ m/s narrows to $A_2 = 1.0\times10^{-3}$ $m^{2}$. New speed (m/s)?

Narrowing Reasoning 🔍

Exit Quiz — Pipe Narrowing ✅

🌳 Branching & Merging Pipes

Part 4 of 7 — Fluids: Continuity

Real plumbing systems split and merge — think household water mains feeding multiple pipes, or arteries branching into capillaries. Continuity still applies: the total volume flow rate in must equal the total volume flow rate out at every junction.

In this lesson you will learn:

• Conservation of $Q$ at junctions: $\sum Q_{in} = \sum Q_{out}$
• Splitting flow between two branches
• The cardiovascular analogy (slow blood flow in capillaries)
• Common AP setups

Junction Equation

At any junction in an incompressible fluid network:

$\boxed{\sum Q_{in} = \sum Q_{out}}$

In terms of areas and speeds:

$\sum A_{in,i}\, v_{in,i} = \sum A_{out,j}\, v_{out,j}$

Splitting Example

A main pipe ($A_0$, $v_0$) splits into two branches ($A_1$, $v_1$ and $A_2$, $v_2$):

$A_0 v_0 = A_1 v_1 + A_2 v_2$

Merging Example

Two streams ($A_1, v_1$ and $A_2, v_2$) merge into one:

$A_1 v_1 + A_2 v_2 = A_3 v_3$

Cardiovascular System

The huge total capillary area means very slow flow — perfect for nutrient/gas exchange. This is continuity in biology!

Branching Concepts 🎯

Branching Calculations 🧮

1. A 0.040 $m^{2}$ pipe splits into two: $A_1 = 0.020$ $m^{2}$ ($v_1 = 4$ m/s) and $A_2 = 0.030$ $m^{2}$ ($v_2 = 2$ m/s). Speed in main pipe (m/s)?

2. Main pipe area 0.020 $m^{2}$, speed 6.0 m/s, splits into two equal-area branches each 0.010 $m^{2}$. If one branch carries 0.040 $m^{3}/s$, the speed in the other branch (m/s)?

3. Three streams ($Q_1 = 0.050$, $Q_2 = 0.030$, $Q_3 = 0.020$ $m^{3}/s$) merge into a pipe of area $0.025$ $m^{2}$. Outlet speed (m/s)?

Branching Reasoning 🔍

Exit Quiz — Branching ✅

⚖ Mass Flow Rate

Part 5 of 7 — Fluids: Continuity

Volume flow rate $Q = Av$ tells you how many cubic meters pass each second. Multiplying by the density turns it into mass flow rate $\dot m = \rho A v$: how many kilograms pass each second. Continuity is really a statement that mass is conserved (EK 8.4.A.1–8.4.A.2).

In this lesson you will learn:

• The definition $\dot m = \rho A v = \rho Q$
• Why conserving mass and conserving volume say the same thing for an incompressible fluid
• How to convert between mass and volume flow rates
• AP-style problems with different fluids

Mass Flow Rate

$\dot m = \rho\, A\, v$

• $\dot m$: mass flow rate (kg/s)
• $\rho$: fluid density $(kg/m^{3})$
• $A$: cross-sectional area $(m^{2})$
• $v$: speed (m/s)

Continuity as Conservation of Mass

Matter cannot pile up inside a completely filled pipe, so the mass entering each second equals the mass leaving each second:

$\rho A_1 v_1 = \rho A_2 v_2$

An ideal fluid is incompressible (EK 8.1.A.4), so $\rho$ is the same everywhere and cancels, giving $A_1 v_1 = A_2 v_2$. Conserving mass flow rate and conserving volume flow rate are the same statement.

Why It Matters

• Mass flow rate answers "how many kilograms per second?", which is what you need for questions about delivered mass or momentum carried by a stream.
• Two different fluids with the same $Q$ carry different mass flow rates: oil ($\rho = 850$) carries less mass per second than water at the same $Q$.
• AP Physics 1 treats every fluid as ideal and incompressible, including slowly moving air in a duct.

Quick Conversions

$Q (\text{m}^3/\text{s}) \times \rho (\text{kg/m}^3) = \dot m (\text{kg/s})$

For water ($\rho = 1000$): $\dot m = 1000\, Q$.

Mass Flow Rate Concepts 🎯

Mass Flow Calculations 🧮 ($\rho_w = 1000$, $\rho_{oil} = 850$)

1. Pipe area $A = 0.0040$ $m^{2}$, $v = 5.0$ m/s carries water. $\dot m$ (kg/s)?

2. Same pipe carrying oil instead. $\dot m$ (kg/s)?

3. A 0.50 $m^{2}$ pipe with $v = 2.0$ m/s carries air ($\rho = 1.2$). $\dot m$ (kg/s)?

Mass Flow Reasoning 🔍

Exit Quiz — Mass Flow Rate ✅

🛠 Continuity Problem-Solving Workshop

Part 6 of 7 — Fluids: Continuity

This workshop combines volume flow rate, pipe narrowing, branching, and mass flow rate. AP problems often require translating words ("the diameter is halved" or "the river widens") into the right ratio.

Workshop Strategy:

1. Write down what's given: $A$ or $r$ or $d$? Speed or volume rate?
2. Use $A = \pi r^2$ for circular pipes (or $A = \pi d^2/4$).
3. Apply $A_1 v_1 = A_2 v_2$ (or junction sums).
4. For mass flow questions, multiply by $\rho$.

Workshop Quick Reference

Strategy Tips

• "Diameter halved" or "radius halved": speed × 4.
• "Pipe doubles in diameter": speed × 1/4.
• "Splits into N equal branches with same cross-section as the main": speed in each branch = $v_{main}/N$.
• "Splits into N equal branches with same TOTAL cross-section": speed in each branch = $v_{main}$.

Workshop MC 🎯

Workshop Calculations 🧮 ($\rho_w = 1000$)

1. A circular pipe of radius 0.020 m carries water at 2.0 m/s. Volume flow rate $(m^{3}/s)$?

2. Same pipe narrows to radius 0.010 m. New speed (m/s)?

3. Pipe area $0.0050$ $m^{2}$, $v = 4.0$ m/s. Mass flow rate of water (kg/s)?

Workshop Reasoning 🔍

Exit Quiz — Workshop ✅

🎯 Synthesis & AP Review — Continuity

Part 7 of 7 — Fluids: Continuity

You've mastered volume flow rate, single-pipe continuity, branching, and mass flow rate. AP loves multi-step continuity questions that lead into Bernoulli (Topic 4), so this synthesis solidifies the foundation.

Big Ideas Recap:

• $Q = Av$ defines volume flow rate $(m^{3}/s)$
• Incompressible continuity: $A_1 v_1 = A_2 v_2$
• Junctions: $\sum Q_{in} = \sum Q_{out}$
• Mass flow rate: $\dot m = \rho Av$

AP Continuity Cheat Sheet

Common AP Question Stems

1. "Speed in a constriction" → $v_2 = v_1 (A_1/A_2)$.
2. "Total flow rate at a junction" → $Q_{out} = Q_{in,1} + Q_{in,2}$.
3. "Hose with thumb partially covering the end" → small $A$, large $v$.
4. "Blood flows slowest in capillaries because..." → huge total cross-section.
5. "Stream of falling water narrows because..." → gravity speeds it up; continuity shrinks $A$.

AP Synthesis MC 🎯

AP Synthesis Calculations 🧮 ($\rho_w = 1000$)

1. A pipe of inner diameter 8 cm carries water at 1.5 m/s. Volume flow rate ($m^{3}/s$, 4 sig figs)?

2. Same flow enters a section of diameter 4 cm. New speed (m/s)?

3. Two pipes ($A_1 = 0.005$ $m^{2}$, $v_1 = 4$ m/s and $A_2 = 0.003$ $m^{2}$, $v_2 = 5$ m/s) merge into a pipe of area $0.010$ $m^{2}$. Outlet speed (m/s)?

AP Concept Synthesis 🔍

Exit Quiz — AP Synthesis ✅