Definitions of few physics terms

[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 believe 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?

 

[Kurdt]

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.

 

[sir-pinski]

Hi there,

I'll do my best to help clarify these definitions for you.

Specific gravity is indeed a relative quantity that compares the density of a substance to the density of water at a specific temperature. This is typically done at 4°C because that is when water reaches its maximum density. So, for example, the specific gravity of iron would be its density divided by the density of water at 4°C. However, this definition can also be used for any other substance, not just water. For example, the specific gravity of gold would be its density divided by the density of water at 4°C. So, you are correct in thinking that specific gravity is always defined as rho/1000 if rho is in kg/m3.

Regarding the specific gravity of water at 0°C, you are correct in thinking that it is not 1.0. The specific gravity of water at 0°C is actually 0.9998, which is very close to 1, but not exactly. This may have been a mistake in the table you found.

As for specific weight, it is defined as the weight of a substance per unit volume. So, it is essentially the same as density, but instead of using mass, it uses weight. The practical benefit of using specific weight is that it allows for easier comparison of substances with different gravitational forces. For example, if you wanted to compare the weight of a substance on Earth to its weight on the moon, using specific weight would be more useful than using density.

I hope this helps clarify these definitions for you. Let me know if you have any other questions.