Every time we go diving we can witness the affects of physics at work and have witnessed them many times with out putting a second thought into them. Many of the concepts that are on the Divemaster's physics exam were taught in Open Water, but to a much watered down degree. For me it was easier to list what each Law did in layman's terms and then learn the math behind it. For me it was a much more logical approach.
Archimedes' Principle - Buoyancy
Archimedes Principle states
"a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid."
Basically to find out how buoyant an object is you need to know, 1. How dense fresh and salt water are by volume, 2. How much an object weighs, and 3. How much volume of water does an object displace in the water?
1 gallon of salt water weighs approximately 8.6 pounds
1 gallon of fresh water weighs approximately 8 pounds
1 cubic foot of salt water weighs approximately 64 pounds
1 cubic foot of fresh water weighs approximately 62.4 pounds
It should be noted that the water density measurements are of water at 62oF. The weights will change slightly with different water temperatures, but that is not important for the physics exam. It assumes that the water density is static.
So if you have an object that displaces 3 cubic feet of salt water and weighs 180 pounds. How would you determine how much weight you would need to add to the object to make it neutrally buoyant in the water?
First you would find out how much "up force" the water is applying with 3 cubic feet. So you would multiply 3 x 64 to get the "up force" of 192 pounds. If the object only weighs 180 pounds and you want it to be neutrally buoyant then you would subtract 192 from 180 to get the difference between the two numbers. 12 pounds of weight would be needed to make the object neutral. Or if you wanted to make the object negatively buoyant then all you would need to do is add 13 pounds of weight instead of 12.
It's a similar concept when you want to find out how much air need to add to a lift bag to make an object buoyant. The concept here is to find out how much water you need to displace with air.
Boyle's Law - Measurement of volume with applied pressure
This is a simple law that the concept was taught in your Open Water class. My OW Manual (SSI) gave an example where it displayed full AL80 at sea-level and taking it down to 33fsw and showing that the volume of the gas in the AL80 was now 40 cubic foot, even though the pressure in the tank still read 3000psi. What this example showed was the effects of the pressure of the atmosphere at sea-level also the pressure of the water. We know that every 33fsw essentially doubles the pressure. Given this we can determine the formula is
P1 x V1 = P2 x V2.
So if the pressure at sea-level is 1 Atmosphere (P
1) and we have a AL80 full of air (lets assumes it's 80 cubic foot). (V
1)
If we take that tank down to 33fsw it is essentially 2 Atmospheres of pressure exerted on the tank. 1 x 80 = 2 x V
2. Given that formula we can assume that the V
2 is 40.
Charles' Law - Measurement of volume with applied temperature
I'm sure everybody here has witnessed Charles' Law in action many times. For example, we put our gear together and check our pressure in our tank before we go get our exposure protection on. Let's say that we are using an AL80 filled to 3000psi. We leave the tank in the hot sun while we go get changed. When we come back we notice that the tank pressure reads 3300psi and then when we get into the water and the tank cools down it goes back down to 3000psi. This is Charles' Law in effect.
The formula to figure out the change in volume is
V1 / T1 = V2 / T2.
For example: We have an AL80 tank filled to 3000psi. (assume that it's 80 cubic foot of air) You can determine that there is 37.5 pressure per square inch (psi) per cubic foot (cf) of gas. (3000/80 = 37.5)
Let's say that the air temperature is 100
oF. So 80cf is V
1 and 100
oF is T
1. If the water temperature is 70
oF (T
2), then what would be our V
2?
80 / 100 = V
2 / 70
V
2 = 56
Given that number we could determine that the pressure in the tank would drop to 2100 psi (56*37.5).
Dalton's Law - Partial pressures
Dalton's Law is perhaps the easiest to remember since it really only deals with simple addition and subtraction. For instance, We know that the air that we breath contains 20.9% oxygen, 79% nitrogen, and .1% other gases such as argon, xenon, etc. Dalton's Law of Partial Pressures simply takes these pressure and adds them together for the total pressure. So 20.9 + 79.0 + .1 = 100% (
P1 + P2 = P3)
Henry's Law - Dissolved gas
Henry's Law really comes into play when you start thinking about decompression theory and how inert gases such as helium and nitrogen dissolve into the tissues while diving. I don't believe that their were any questions on the exam about henry's law.
