# Fluid Flow

## Key Questions

• Bernoulli's Principle can be seen in various places around us.

1. Fast moving train passing the platform
a. When a fast moving train rushes past the train platform, the velocity of the air particles next to the train increases. This leads to a decrease in the air pressure around the train.
b. If one is standing very close to the edge of the platform, they can be harmed as the air will move from the high pressure area behind them to the low pressure area created in front of them by the fast moving train.

2. Spinning of a ball in cricket/baseball/football

Observe the figure carefully.
1. The air travels faster relative to the center of the ball where the periphery of the ball is moving in the same direction as the airflow (left). This reduces the pressure, according to Bernoulli's principle.
2. The pressure increases on the other side of the ball, where the air travels slower relative to the center of the ball (right).
There is therefore an imbalance in the forces, and the ball deflects in the same sense as the spin - from bottom right to top left.

• A river flowing down a mountain; air passing over a bird's wing; blood moving through a circulatory system; fuel moving through an engine. These are all examples of fluid flow.

A fluid is a substance that conforms to the shape of whatever vessel contains them. For instance, if you take water from a jar and pour it into a tea kettle, it will change from a jar shaped mass to a tea kettle shaped mass.

In this sense, any liquid or gas can be classified as a fluid, and this article defines fluid flow as, "motion of a fluid subjected to unbalanced forces or stresses."

Imagine a jar full of water tipped over. The fluid at the bottom of the jar is held up against gravity, and therefore does not fall. The forces acting on this fluid are balanced. The water at the top, however, is not balanced and is free to move sideways and over the lip of the jar, therefore flowing toward the ground.