Molasses Madness


Model Description

This is a demonstration of the concept of conservation of mass.  By pouring an incompressible fluid (molasses), it is clearly demonstrated that as the velocity of the flow increases, the cross-sectional area of the flow must decrease. This demonstration should take 8-10 minutes.

Engineering Principle

Mass is conserved when the mass flow rate into the system equals the mass flow rate out of the system:

\dot{m_{in}} = \dot{m_{out}}

The mass flow of a fluid at any point is the product of the density, cross-sectional area and flow velocity.  Therefore, we can rewrite the conservation of mass between points one and two as:

\rho_1A_1V_1 = \rho_2A_2V_2

The density of molasses is assumed constant between points one and two and equation (2) reduces to:

A_1V_1 = A_2V_2

If the cross-sectional area of the flow decreases (A_2 < A_1 ), then the flow velocity must increase to maintain the conservation of mass (V_2 > V_1 ).

What You Need

Item Quantity Description/Clarification
Jar of Molasses 1 Any kind will do.
Beaker 1 Any kind of cup will suffice to collect the molasses
Wet Paper Towels 5-10 Used for the inevitable mess

How It’s Done

Before Class: Open the jar of molasses as it will inevitably be stuck shut otherwise during class.

In Class: After a discussion of the conservation of mass, pour the molasses from the jar or a cup into the beaker.  The cross-sectional area of the flow initially (A_1 ) is large and the flow velocity (V_1 ) is low.  As the flow accelerates due to gravity, the downstream velocity (V_2 ) increases (figure below, bottom left). As this velocity increases we see the cross-sectional area (A_2 ) decrease (figure below, bottom right).

Observations:  Students should be able to observe how the cross-sectional area decreases as the velocity increases.

Additional Application: Ask the students for common examples where we increase the flow velocity by decreasing the area (i.e. shower head, squirt gun, etc.). Using these examples, draw a schematic of the device illustrating the conditions at points one and two.

Did you try this? Comment below to let us know how it went.

Cite this work as:

Phil Root (2019), "Molasses Madness,"

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