What is a Fluid?
When we think about motion, we usually imagine objects moving through air or water. Both of these are examples of fluids — and understanding how objects move through them is essential to explaining everything from skydiving to submarine navigation.
Any substance that can flow and take the shape of its container. Fluids include both liquids (like water and oil) and gases (like air). The key property of a fluid is that its particles can move past one another freely.
You interact with fluids every single day. When you walk outside, you move through air. When you swim, you push through water. Even sitting still, the atmosphere pushes against your skin. Fluids are everywhere, and they always exert forces on objects moving through them.
Think about cycling on a calm day versus cycling into a strong headwind. The wind is just air — a fluid — but it makes a huge difference to how hard you have to pedal. That resistance you feel is the force we call drag.
Drag: The Resistance Force
Whenever an object moves through a fluid, the fluid pushes back against it. This opposing force is called drag.
A force that opposes the motion of an object moving through a fluid (liquid or gas). Drag always acts in the opposite direction to the object's motion.
Drag is caused by two main effects:
- Friction between the fluid and the surface of the object (sometimes called skin friction)
- Pressure differences — the fluid has to part in front of the object and rejoin behind it, creating a high-pressure zone at the front and a low-pressure zone at the rear
The result is a net backwards force that tries to slow the object down.
Drag is sometimes called air resistance when the fluid is air, and water resistance when the fluid is water. These are all the same type of force — just in different fluids.
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