# Dimensions and Dimensional analysis question

1. Oct 26, 2013

### Zerocool97

In this formula, E = 1/2 mv²

1/2 is not considered as it does not have any dimension. But then does it not affect the formula. Why? Explain!! Please

2. Oct 26, 2013

### Simon Bridge

Welcome to PF;
Of course the value of 1/2 affects the formula... what is there to explain?

3. Oct 26, 2013

### dauto

That's a limitation of dimensional analysis. numerical factors such as 1/2 in the formula for the kinetic energy cannot be calculated by dimensional analysis. Some more advanced calculation method must be used.

4. Oct 26, 2013

### SW VandeCarr

Dimensional analysis is generally done with a few chosen "basic" dimensions. For most problems in Newtonian mechanics, length, mass and time (LMT) are sufficient. For energy, dimensions are: $E=ML^{2}T^{-2}$. The dimensional formula does not distinguish between kinetic and potential energy. Dimensionless values such as 1/2 in the KE formula do not appear in the dimensional formula.

Also, dimensional formulas do not distinguish between energy density and pressure. Both are represented by the formula $ML^{-1}T^{-2}$. The difference is that energy density is a scalar quantity and pressure is a vector quantity.

DA is useful in showing relationships between different formulas used in physics. That doesn't mean that different formulas with the same dimensional formula are necessarily strictly equivalent. They may only be dimensionally equivalent.

Last edited: Oct 27, 2013