# Supposedly Simple Volume Problem

#### cocokat

A typical adult human lung contains about 330 million tiny cavities called alveoli. Estimate the average diameter of a single alveolus. Assume the alveoli are spherical and a typical human lung is about 1.9 liters.

The answer needs to be in meters.

This is what i did: I converted 1.9 L to .0019 m^3, and divided that by 330 million to get 5.757575*10^-11 m^3. Then I set this to 4 pi r^3 over 3 and found the radius, which i multiplied by 2 to get a diameter of 1.03*10^-11 which was wrong. Ive also done this calculation to get an answer of .00048 which was also wrong. Can anyone help?

#### cocokat

Also...

A rock is dropped from a sea cliff and the sound of it striking the ocean is heard 5.4 s later. If the speed of sound is 340 m/s, how high is the cliff?

I thought that if u multiply 5.4 by 340 to get 1836 m that the sound traveled that would help but now I'm just stuck. That couldn't be the distance the rock traveled otherwise itd b the height of the cliff (its not). Really all I know is that the initial velocity is 0. Now what?

P.s. Sorry for all the questions it's my first day in AP Physics and its not going well #### werehk

PHF Hall of Fame
Note that time is needed for rock to fall. You have to use the equation of motion to calculate how long it takes for the rock to reach the ocean. 5.4s includes time needed for the rock to reach the ocean and time needed for sound to travel from ocean to observer's ear.

#### ChipB

PHF Helper
This is what i did: I converted 1.9 L to .0019 m^3, and divided that by 330 million to get 5.757575*10^-11 m^3.
Check your math, specifically the exponent - you should get 5.76 x 10^-12 m^3.

Then I set this to 4 pi r^3 over 3 and found the radius, which i multiplied by 2 to get a diameter of 1.03*10^-11 which was wrong.
The cube root of a number on the order of 10^-12 will be on the order of 10^-4, so this is clearly wrong.

Ive also done this calculation to get an answer of .00048 which was also wrong. Can anyone help?
If you fix the first issue you should get the correct answer.