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1.9: Dimensional Analysis

JoVE Core
Mechanical Engineering

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Dimensional Analysis

1.9: Dimensional Analysis

Dimensional analysis is a powerful tool that is used in physics and engineering to understand and predict the behavior of physical systems. The basic idea behind dimensional analysis is to express physical quantities in terms of fundamental dimensions such as the mass, length, and time. Derived dimensions like the velocity, acceleration, and force are derived from the combinations of these fundamental dimensions.

Dimensional analysis allows us to analyze and compare physical quantities on a more fundamental level, independent of the specific units used to measure them. It involves assigning fundamental dimensions to physical quantities and checking if the equations satisfy dimensional homogeneity. If the powers of the fundamental dimensions on both sides of an equation are identical, the equation is considered dimensionally homogeneous. Dimensional analysis also allows us to check the validity of equations and derive new equations by comparing the units on both sides. Dimensional analysis helps to identify mathematical errors and improve the understanding of the underlying physical relationships described by the equations.

For example, consider a ball being hit by a bat. The force applied by the bat is calculated as the product of the mass and acceleration. Acceleration is simply the change in velocity per unit time, while velocity is the change in displacement over time. By expressing the force in terms of these fundamental dimensions, we can compare the force required to hit balls with different masses or under different conditions.

Several methods are used in dimensional analysis, including Rayleigh's method and Buckingham's pi theorem. Rayleigh's method is used to determine the expression for a variable that depends on a maximum of three or four independent variables. This method is often used to derive empirical equations that can be further used to describe physical systems. On the other hand, Buckingham's pi theorem states that if there are n variables in a dimensionally homogeneous equation containing m fundamental dimensions, they may be grouped into (n m) dimensionless terms. This theorem is widely used to analyze fluid mechanics and other areas of physics and engineering.

Suggested Reading


Dimensional Analysis Physics Engineering Fundamental Dimensions Mass Length Time Velocity Acceleration Force Dimensional Homogeneity Equations Units Of Measurement Mathematical Errors Physical Relationships

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