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More Than 475 Free MCAT Questions with Detailed Answers!

## Physics - Fluids in Motion

Think about fluid flowing in a cylindrical pipe of uniform radius. What makes the fluid flow is a pressure difference between the ends of the pipe. These things tend to slow down the flow of fluid.

Narrowness of the pipe :
A thin pipe impedes fluid flow.

Length of the pipe :
The sides of the pipe impose frictional force on the fluid. The longer the pipe, the greater the force.

Viscosity :
This is the cohesive molecular forces in a fluid. Greater viscosity means greater frictional resistance to flow. Viscosity tends to decrease with increasing temperature.

Turbulence :
Fluid flow becomes turbulent at speeds above a certain level. In turbulent flow, eddies and whirlpools create frictional force that impedes the flow. The opposite of turbulent flow is laminar or streamline flow. There is a formula that describes these relationships, but you do not have to know it. However, you should know some of the quantitative relationships that arise from this formula.

Rule number 24: Fluid flow

When fluid flows in a length of cylindrical pipe;All other things being equal flow at every point in the pipe is directly proportional to the pressure difference between the two ends of the pipe.

All other things being equal flow at every point in the pipe is greater if the radius of the pipe is greater and less if the radius of the pipe is less.
Flow is inversely proportional to the fluid’s viscosity.
Flow is inversely proportional to the length of the pipe.
All other things being equal, flow is less if it is turbulent and greater if it is streamline.

You also need to know something about ideal flow. Ideal flow is flow of an imaginary fluid that has no viscosity to impede its flow is completely incompressible and flows in a streamline fashion.

Rule number 25:
At any point in a system of continuously flowing fluid, flow is equal to area times fluid viscosity.

There is one more thing you should know about ideal flow in a pipe of uniform radius. The pressure is lowest at points of highest elevation and greatest at points of lowest elevation.

It is important also that you know a few things about fluid that is flowing in a system whose pipes or channels vary in size. First, in a single system of continuously flowing fluid, flow is the same at all points. Second, at any point in the system of continuously flowing fluid, flow at any point in the system is equal to the cross-sectional area of the pipe times the velocity of the fluid. Third, if a system is at equal elevation at all points, then at areas of wider caliber, pressure is higher and velocity is lower. At areas of narrower caliber, pressure is lower and velocity is higher.

Summary of fluids in motion

Actual flow:
1. Within a length of cylindrical pipe of uniform dimension, flow between any two points is directly proportional to pressure difference between the two ends.
2. Within a length of cylindrical pipe of uniform dimension, flow between any 2 points is greater with greater radius and lesser with lesser radius.
3. Within a length of cylindrical pipe of uniform dimension, flow is inversely proportional to pipe length.
4. Within any system of flow, flow between any two points is inversely proportional to fluid viscosity.
5. Within any system of flow, flow between any two points decreases with increased turbulence.
Ideal flow:
1. In a system of ideal flow, flow equals area times velocity.
2. In a system of ideal flow, flow is the same at all times.
3. In any system of ideal flow, area and velocity are inversely proportional, which means that if area is doubled, velocity is halved. If velocity is doubled, area is halved.
4. In a system of ideal flow, reduced area produces lower pressure and greater velocity. Increased area produces greater pressure and lower velocity.