Physics

[agwatts]

I took my Physics exam last night and failed.

Does anyone have any tips, other than study night and day, to the keys in passing this blasted test?

I'm really bummed and

Greg

 

[Paco]

I wouldn't study night and day. Study in several shorter sessions. It's all about the brain's saturation point and I believe we retain more by studying several short sessions than fewer long ones.

Also, remember to bring objects to the surface and remember to add 1. If you know what I mean by that you should be close.

 

[CDNScubaMoose]

Can you give us an idea as to which part of the physics was giving you trouble? Was it the calculations questions or the theory ones (or maybe both)?. Also - which agency?

 

[diverdowndh]

Ouch, I am probably going to do this exam next after taking the equipment and environment tests tomorrow night. I believe we are going to schedule physics for next Thursday night.... I'll definitely try to be prepared for it after this premonition.
Bummer Greg!

 

[Joe-Diver]

This is the one that really frustrates most folks. From my study sticky above:

This is the one that most people sweat about and for most it is the hardest test. Read chapter 4 in the Encyclopedia and then complete section 1 in the Workbook. Understand the gas laws and know how to use the formulas. When you have a question that starts at one depth and asks you to solve for another depth, always take it to the surface, then back down. Make notes for yourself, defining the gas laws and formulas in the simplest terms you understand so you can quickly review your notes right before the exam.

Work the problems, understand the concept behind the formulas. If you can get a picture in your head "why" this works the way it does, you can grasp the concept. Once you do that, they can ask you any question in any way and you'll be able to answer it.

Don't forget: You can always ask your Instructor for a class session!

I know how much they love to do knowledge elaborations on Divemaster Physics.

 

[CDNScubaMoose]

When I was prepping for my IDC last winter, I was worried about the physics part. I am pretty good with the math part but I was worried about trying to memorize the formula's and other constants. My approach was to make sure I understood the concept - ex. gas expands as pressure goes down so therefore if I have a pressure/volume question then if the depth increases then my answer better be a lower volume - that was my first step to make sure my answer made sense. The next step for me was to practice doing the problems, I would often write my own problems and then attempt to solve them. I was doing the IDC with a friend so we would often email each other problems and check each others answers. Do you have a classmate that you can do that with, or is your instructor willing to make up more problems for you to answer?

 

[AzAtty]

As others have said, it would be helpful to know what parts challenged you. If you post the specific issues, we could run you through some sample problems.

Here is what to remember for what most consider the "hard" parts of the physics exam:

1. General gas law: (P1xV1)/T1 = (P2xV2)/T2. The general gas law is a combination of Charles' law, Guy-Lussac's law, and Boyle's law. To use the formula, substitute given pressures, volumes, and temperatures to derive an answer. If a variable is not given, then it is either (a) constant (meaning you can eliminate it from the equation) or (b) it is the variable you are trying to find.

2. Archimedes principle (an object is buoyed up by a force equal to the weight of the liquid displaced; objects of equal volume are buoyed up by an equal force). It's helpful to think of "up force" and "down force." The "up force" is the weight of water displaced by an object. The "down force" is the weight of the object. So if a question asks you how to make something positive, you calculate the amount of water to displace to give it a greater "up force" than "down force." Likewise, if a question asks you how to make something "X lbs" negative, you calculate the amount of weight you must add to increase the "down force" so it exceeds the "up force" by "X lbs."

3. Calculations using the general gas law are always expressed in absolute pressure. The surface is zero psi GAUGE, but 1 atmosphere absolute. A common error is to omit atmospheric pressure at the surface from calculations involving pressure at depth.

4. "Taking it to the Surface": When presented with a question that gives preliminary information at one depth, and then requests an answer for a variable at another depth, you must make an "intermediate" calculation of the condition of the variables at the surface before calculating the condition of the variables at a second given depth.

5. Gas consumption questions are just a variation on a theme. All you have to do is divide or multiply a given rate of consumption by absolute pressure. Remember to take it to the surface first.

 

[ClayJar]

1. General gas law: (P1xV1)/T1 = (P2xV2)/T2. The general gas law is a combination of Charles' law, Guy-Lussac's law, and Boyle's law. To use the formula, substitute given pressures, volumes, and temperatures to derive an answer. If a variable is not given, then it is either (a) constant (meaning you can eliminate it from the equation) or (b) it is the variable you are trying to find.

I know of many people who tried their hardest to memorize all the different gas laws when it seems ever so much less confusing when you start from the general gas law and then just toss the parts that aren't in the problem. When I'm helping people, I most often try to give them a "holistic" grasp of the general gas law, but then again, I only bothered remembering PV=nRT back in high school and college.

Oh, and I know it was just a typo, but you had V₂ twice. Lest anyone become confused by the typo, it's:

P₁ * V₁ / T₁ = P₂ * V₂ / T₂​

(Also, I'd gladly offer to go to the ScubaBoard chat sometime to help anyone with grasping physics.)

 

[AzAtty]

Thanks for the catch. At least we don't have to memorize Avogadro's number.

 

[CDNScubaMoose]

At least we don't have to memorize Avogadro's number.

6.023 x 10 (to the power of 23) right? Been a while but a guess once a chemistry geek, always a chemistry geek.