What exactly is Density and what am I calculating?

  • Thread starter semidevil
  • Start date
  • Tags
    Density
In summary, the CDF is a function that tells you how many objects are in a given area, and the density is the derivative of the CDF.
  • #1
semidevil
157
2
What exactly is "Density" and what am I calculating?

So I"m doing a lot of probability problems with CDF and although I know how to calculate it, I don't quite know what I am calculating (in terms of relating it to real life situations).

Example:
lets say the lifetime of a machine part has a continuous distribution of (0,40) with a density function that is proportional to (10x+2)^-2. What is the probability that the lifetime of a part is less than 5.

So I know how to solve this: integrate, find the density, and we get a value for C. Then, we integrate again to between 0 and 5 to get the probability that it is less then 5.

First of all, what is (0, 40) when they say there is a distribution between that? what does it mean that the probability of less than 5 is 5/12 (the correct answer)?

since 5 is on the lower end of (0, 40), is it saying that the part has a very low lifetime?
 
Physics news on Phys.org
  • #2
semidevil said:
Example:
lets say the lifetime of a machine part has a continuous distribution of (0,40) with a density function that is proportional to (10x+2)^-2. What is the probability that the lifetime of a part is less than 5.

So I know how to solve this: integrate, find the density, and we get a value for C. Then, we integrate again to between 0 and 5 to get the probability that it is less then 5.

First of all, what is (0, 40) when they say there is a distribution between that?
It means that P(0<x<40)=1
For your example, the part will certainly fail before it turns 40, but after it leaves the production line (at t=0).

what does it mean that the probability of less than 5 is 5/12 (the correct answer)?
That means that there is a 5/12 probability that the part won't turn 5.
You would expect, in a big production run, that 5 out of every 12 parts will have failed by 5 time-units after they were made.

since 5 is on the lower end of (0, 40), is it saying that the part has a very low lifetime?
That depends - if the required lifetime is only 1 time-unit, this is a long time.
It just means you'd expect almost half of them not to be really long lived.

Think cars - with careful maintenance a car can last for a century or more, but most get trashed before they are a decade old and you are lucky to get a guarantee longer than 18 months. Does that mean cars don't last very long?
 
  • #3
semidevil said:
So I"m doing a lot of probability problems with CDF

You mention "density" in the title of your post. Do your materials call the CDF a "cumulative density function"? Many texts call the CDF a "cumulative distribution function". It's derivative is the "density".
 

FAQ: What exactly is Density and what am I calculating?

What is density?

Density is a measure of the amount of mass per unit volume. It is commonly denoted by the symbol "ρ" and is calculated by dividing the mass of an object by its volume. In simpler terms, density is how much "stuff" is packed into a certain space.

How is density calculated?

Density is calculated by dividing the mass of an object by its volume. The formula for density is: ρ = m/V, where ρ is density, m is mass, and V is volume. The units for density are typically expressed in grams per cubic centimeter (g/cm3) or kilograms per cubic meter (kg/m3).

Why is density an important concept?

Density is an important concept because it is a fundamental property of matter. It helps scientists identify and classify different substances, as each substance has a unique density. Density is also important in many practical applications, such as determining the buoyancy of objects, predicting how substances will interact, and even in medical diagnoses.

What are some common units of density?

The most commonly used units of density are grams per cubic centimeter (g/cm3) and kilograms per cubic meter (kg/m3). However, depending on the context, other units such as pounds per cubic inch (lb/in3) or ounces per gallon (oz/gal) may also be used.

How does density relate to the physical properties of a substance?

Density is a physical property of a substance, meaning it is a characteristic that can be measured and observed without changing the identity of the substance. The density of a substance is determined by its molecular structure and the arrangement of its particles. For example, substances with tightly packed particles tend to have a higher density than substances with loosely packed particles.

Similar threads

Replies
11
Views
2K
3
Replies
72
Views
2K
Replies
1
Views
1K
Replies
2
Views
1K
Replies
7
Views
2K
Replies
1
Views
776
Back
Top