SI Unit Conversion Help: Mass Density of Aluminum and Alcohol

In summary: SI units. You went to go multiply them together to find an area and you found that your area was not in SI units.In summary, the book says that the quantity called mass density is mass per unit volume of a substance. The book provides two examples of how to convert between SI and English units.
  • #1
Weave
143
0

Homework Statement


Ok, this isn't hard, I'm just thrown off by the question. From the book: "The quantity called mass density is mass per unit volume of a substance. Express the following in SI units:"

a. Aluminum, 2.7*10^-3 kg/cm^3
b. Alcohol, .81g/cm^3



Homework Equations





The Attempt at a Solution


So do they want it just in g/kg?
A. multiply by cubic centimeters per kg
B. multiply by cubic centimeters per g
 
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  • #2
SI units don't simply mean "metric". There are very specific metric quantities that are considered SI units. For example, the SI unit for temperature is the Kelvin. If you were given a Celsius measurement, you'd have to convert that to Kelvin if you wanted to express the temperature in an SI unit.

You can see which units are SI units from this link:

http://en.wikipedia.org/wiki/SI

For example, in part a of your question, you're given kg and cm^3. One of those is SI and the other isn't.
 
  • #3
SI units are also sometimes referred to as "MKS" units- the fundamental units of length mass and time are "Meter", "Kilogram", and "Second"- unit of force is the Newton, energy the Joule, etc. This is as opposed to the "cgs" system in which the fundamental units of length, mass, and time are "centimeter", "gram", and "second"- the unit of force is the dyne, energy the erg, etc.

In any case, your first measurement is 2.7*10^-3 kg/cm^3 which is in neither system. You need to convert grams to kilograms. The second measurement is .81g/cm^3 which is in cgs. You need to convert grams to kilograms to grams and centimeters to meters.

Use the "unit fractions" 1 kg/1000 g and 100 cm/ 1 m.
 
  • #4
The first measurement was already in kg. :-)
 
  • #5
HallsofIvy said:
In any case, your first measurement is 2.7*10^-3 kg/cm^3 which is in neither system. You need to convert grams to kilograms. The second measurement is .81g/cm^3 which is in cgs. You need to convert grams to kilograms to grams and centimeters to meters.

Use the "unit fractions" 1 kg/1000 g and 100 cm/ 1 m.

The first one is already in Kg, so wouldn't I just convert the cm^3 to m^3?
The second one needs to be converted to kg, then cm^3 to m^3.
 
Last edited:
  • #6
Yup! :-)
 
  • #7
m^3 is an SI unit still, right?
 
  • #8
Weave said:
m^3 is an SI unit still, right?

Just imagine how odd it would be if you had two lengths expressed in SI units. You went to go multiply them together to find an area and you found that your area was not in SI units.

I think we'd all go back to using the English system...
 

Related to SI Unit Conversion Help: Mass Density of Aluminum and Alcohol

1. How do I convert between different SI units?

To convert between different SI units, you can use conversion factors. These are ratios that relate one unit to another. For example, to convert from meters to centimeters, you would use the conversion factor 1 meter = 100 centimeters.

2. What is the difference between base and derived SI units?

Base SI units are the seven fundamental units of measurement that form the basis of the SI system. These include the meter, kilogram, second, ampere, kelvin, mole, and candela. Derived SI units are combinations of base units that are used to express quantities of physical quantities. For example, the unit of speed, meters per second, is a derived unit.

3. How do I convert between different prefixes in SI units?

To convert between different prefixes in SI units, you can use the fact that each prefix represents a power of 10. For example, to convert from millimeters to meters, you would move the decimal point three places to the left, since milli- represents 10^-3.

4. How do I know when to use scientific notation in SI unit conversions?

Scientific notation is typically used when working with very large or very small numbers. It is a way to express numbers in the form of a coefficient multiplied by 10 raised to some power. For example, the speed of light in a vacuum, 299,792,458 meters per second, can be expressed as 2.99792458 x 10^8 m/s in scientific notation.

5. Can I use online converters for SI unit conversions?

Yes, there are many online converters available that can help with SI unit conversions. However, it is important to understand the principles behind unit conversions and how to use conversion factors, rather than just relying on a converter. This will help you to better understand and apply unit conversions in your scientific work.

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