# Definitions of few physics terms

1. Nov 26, 2007

### Micko

Hello people,
Please, can you help me understand meaning of few definitions like:
First, there is so called specific gravity, defined as relative quantity:
SG = rho/rho(H2O) at temperature of 4 °C. Since water's density at 4 °C is 1000 kg/m3 = 1 g/cm3, it seems that specific gravity is always rho/1000 if rho is in kg/m3. What I don't know is if this definition is applicable to any substance, so for example, if we speak about SG of iron, it is rho(iron)/1000 if calculated at 4°C. Is this always defined according to ref. tmperature of 4°C or not?
What confuses me is that I found in one table that SG of water at 0°C is 1.0. I belive this is mistake since water's density at 4°C is 1000 kg/m3 and not at 0°C. Is this correct?

Also, specific weight is defined as gamma = rho * g? What is practical benefit of using these quantities? Isn't plain ordinary density sufficient?

2. Nov 28, 2007

### Kurdt

Staff Emeritus
The specific gravity is the relative density of a substance wrt water at 4 degrees C as you have stated. This means the denominator will always be 1000. I'm not sure if the temperature really matters since relative density is just used to compare unknown substances (i.e. compound substances such as rocks) with something that is we defined. Someone can correct me if I'm wrong. The point of the comparison is that it gives one an easy way of finding out the density of something without having to do tricky things like measure its volume.