Flow Pressure


The force applied to a plate and bowl by an airflow are compared. This can be seen as analogous to inelastic and elastic collisions.

Directed flow
Directed flow
Redirected flow
Redirected flow



Principles Illustrated

Force = rate of change of momentum in a fluid.

NCEA & Science Curriculum

Can be used as a starting point for investigations in PHYS 1.1, PHYS 1.2


With a scale: Zero the scale with the bowl and cardboard in place (not done in the example photos below). With the cardboard sheet on top of the bowl air is deflected mostly to the sides. The reading on the scale increases as a force must be applied to change the vertical component of momentum of the fluid to zero. When the cardboard sheet is placed beneath the bowl the air flow is reversed rather than deflected. The momentum change and thus the force are roughly doubled.






A video camera is pointed at the scale for use in a large lecture theatre. This has been tested and works well.

Without a scale: Students can actually feel the difference in the force when the bowl and cardboard are reversed if they hold the bowl on one hand.


Note that the force needed to reflect the air is approximately twice the force needed to deflect the air, as expected.

FlowPressureFlow_Off_smallFlow off: 0.134 kg

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FlowPressureFlow_On_Deflect_smallFlow deflected: 0.315 kg

F = (0.315kg – 0.134 kg) (9.8 m/s^2) = 1.77 N

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FlowPressureFlow_On_Reverse_SmallFlow reflected: 0.463 kg

F = (0.463kg – 0.134 kg) (9.8 m/s^2) = 3.22 N

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The air flow from the blower can be quite substantial and can carry dust and grit with it. Keep the blower pointed well away from faces or wear goggles to avoid potential eye damage.

Individual teachers are responsible for safety in their own classes. Even familiar demonstrations should be practised and safety-checked by individual teachers before they are used in a classroom.

Related Resources

Inelastic and Elastic Collisions

Teaching Resources

Would you like to contribute lesson suggestions? Contact us.


This teaching resource was developed with support from

The MacDiarmid Institute
Faculty of Science, Victoria University of Wellington
School of Chemical and Physical Sciences, Victoria University of Wellington


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