Fluid Mechanics


Liquids and gases are all around us and important for everyday life, from the water we use to shower in the morning… to the hydraulic brakes in the vehicle we take to and from school… to the heating system that keeps our house warm at night. In order to benefit from these fluids, we need to understand how they behave and engineer systems that transport and use them.

Learning objectives:
1. Define a fluid and understand the importance of fluids
2. Learn Bernoulli’s principle and equation
3. Apply Bernoulli’s equation to practical situations


Question: What do these four images have in common?

Hint: What do we call the air rotating a wind turbine, the water flowing down a waterfall, the vapour coming out of a stack, or the blood pumping through an artery?

Answer: They are all fluids!

We all have an intuitive understanding of a fluid – it’s something that flows, like water or air. But what is the formal definition of a fluid?

Definition: A fluid is…

1. a substance that deforms under the action of shearing forces

2. unable to retain a certain shape if unsupported

3. assumed to behave as a continuum, i.e. not as separate objects

In other words, a fluid will flow if it experiences an external force (point #1), it takes the shape of its enclosing container (point #2), and it acts smoothly as a single entity without seeing the motion of individual molecules (point #3).

So why do we care about fluids? Quite simply, because they are all around us!


Can you think of 5 everyday systems that contain or use fluids, in addition to the four above?

Here are some possible answers, although there are many, many others: toilet, building ventilation, ink jet printer, refrigerator cooling, airplane lift, hoover, hydraulic brakes, river, water spouts, power dams, natural gas piping, etc.

In order to design and create processes and systems that control and use liquids and gases efficiently and safely, engineers need to be able to understand, describe, and predict fluid behaviour. This field of science is called fluid mechanics: the study of forces within liquids and gases when they are still (fluid statics) or moving (fluid dynamics).

We’ll look at a fundamental principle that allows properties of a fluid to be calculated and we’ll explore how it can help us understand everyday phenomena and apply it to practical situations. Download the resource activities to further explore this topic.

Video Resource

Video Resource

Resource activities

Bernoulli's Principle

Bernoulli’s principle relates the pressure of a fluid to its elevation and speed. Read this activity before attempting the questions. 



Question sheet applying Bernoulli's Principle.



Answer sheet to the questions.


Reflective questions

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Task 1

What are the key arguments, concepts, points contained within it?

Task 2

What are you struggling to understand?

What could you do to improve your understanding of these concepts/terminology etc.?

Task 3

What further questions has this resource raised for you?

What else are you keen to discover about this topic and how could you go about learning more?

Can you make any links between this topic and your prior knowledge or school studies?

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Further reading