# PADI IDC Theory - Physics

### Test yourself with these questions

## PADI Dive Theory - Physics

I’ve met loads of people who tell me that they’re hopeless when it comes to Physics.

Does that sound like you?

In a moment, I’ve got a few physics practice questions for you to try out.

But for now, let’s find out what you DO know about physics.

It’s easier than you think!

### You already know a lot about physics

You know that you breathe more air the deeper you go.

Balloons get smaller and so does the effect of the gas that you breathe.

Oxygen can become harmful, and Nitrogen can become narcotic.

You already know this stuff.

It’s physics. But it’s just diving too. PADI IDC Theory physics isn’t too difficult!

## PADI IDC Theory Physics

Ok, let’s take things one step at a time.

Here we’ll talk about THREE major topics that come up time and time again on PADI Physics exams.

In each of the areas, I’ll give a few tips, and show you a video.

Then you’ll get a chance to answer some questions.

Each of the videos are from my Online Preparation Course.

You can watch the video first, or you can steam straight into the questions.

Good luck!

## 1. Calculating pressures underwater

#### The Basics

Being able to calculate pressures in seawater is essential.

Without this knowledge, you’ll struggle with some PADI Physics exam questions.

Like where they ask you what size a balloon will be when you take it to 30 metres / 99 feet.

The good news is that it’s easy!

## How it's done

For every 10 metres or 33 feet, the water (or gauge) pressure changes by 1 atm.

The total pressure at depth includes both gauge and atmospheric pressure.

So, pressure at 10 metres or 33 feet = 1 atm for the gauge pressure, and 1 atm = 2 atm.

#### This might help

**Metric**For every one metre in sea water, pressure changes by 0.1 atm. Example: at 23 metres pressure is: 23 X 0.1 = 2.3 atm for gauge, +1 = 3.3 for absolute.

**For every foot of sea water, pressure changes by 0.0303 atm. Example at 76 feet pressure is 76 X 0.0303 = 2.3 atm for gauge + 1 = 3.3 for absolute**

Imperial

Imperial

**Fresh Water**

In fresh water, it’s much the same, but for metric use 0.097 for every metre, and in imperial use 0.0294 for every foot.Effe

## PADI IDC Revision - Physics - Pressures Underwater

OK, let’s test yourself with these questions.

### PADI IDC Exam Questions

What is the gauge pressure at 34 metres / 112 feet of fresh water?

3.3 atm / 48.4 psi

Is the gauge pressure for a given depth in salt water be the same/greater/less than the same depth of fresh water?

Greater than

What is the absolute pressure at 20 metres / 66 feet of fresh water?

3 bar / ata

What is the absolute pressure at 34 metres / 112 feet of fresh water?

4.4 ata / 64.5 psia

How did you get on? Check out this video for lots more information

## 2. PADI IDC Theory: Physics - Effects of Pressure

Let’s move on to Effects of Pressure.

These ones are so important.

Usually PADI Physics exams have over more questions about the effects of pressure than any other subject.

You can go straight to the questions or take a look at this first.

#### The Basics

These questions usually make up around 40% to 50% of all PADI IDC Physics exams. Anything to do with changes at depth: Balloons, Breathing rate, Density, Partial Pressure of gasses, How much air needs to be pumped from the surface etc.

#### How it's done

Watch the video to get a really good tip on how to work these out!

As you get deeper, some things get smaller (e.g volumes) and some things get larger (e.g.density, breathing rates)

First, work out whether the answer will be larger or smaller. That will tell you whether you’ll multiply or divide.

*"A HUGE thank you for the fantastic Distance Learning course (and I do LOVE Physics now!). Your teaching methods are second to none"*

Caroline

### PADI IDC Exam Physics Effects of Pressure Questions

Imagine that the air in your cylinder was accidentally mixed with 0.5% carbon monoxide. If you took this to 20 metres / 66 feet, what would the surface equivalent percentage be?

1.5%

The actual percentage would still be 0.5%

But the effect would be equivalent to surface percentage of 1.5%

Approximately how much air must be pumped down from the surface to fill a 40 litre/1.4 cubic feet container if the container lies in 20 metres/66 feet of sea water?

120 litres / 4.2 cubic feet

Approximately how much air must be pumped down from the surface to fill a 50 litre container if the container lies in 40 metres/132 feet of sea water?

250 litres

If it takes a diver 90 minutes to breathe all of the air from a cylinder at the surface, appoximately how long will the air in that cylinder last at 20 metres / 66 feet of sea water if all of the other conditions remain the same?

30 minutes

A diver is using a cylinder filled with air (21% Oxygen, 79% Nitrogen) With respect to the oxygen, at approximately what depth would breathing this mix have the same effect as breathing pure oxygen at the surface?

40 metres/130 feet

During a wall dive in the ocean, you release a balloon at a depth of 24 metres/80 feet. The balloon contains one litre of air. What is the balloon’s volume at the surface?

3.4 litres

*“I have to say your Physics formulas helped me no end, it all made sense! Amazing, I can’t believe I now understand Physics, so a big THANK YOU to you for the preparation course"*

Andrea

## 3. PADI IDC Revision - Physics - displacement

Now we’ll move on to the displacement topic.

This is the Archimedes principle, where back in ancient Greek times He stated: **a body immersed in a fluid experiences an upthrust equal to the weight of the fluid displaced.**

Which simply means that if you jumped into the sea, with all your kit on, water would be displaced.

Now, IF the weight of that displaced water weighed the same as you – well, you’d be in luck! because that would mean that you’d be neutrally buoyant!

*"The Distance Learning preparation course made me feel so more relaxed about the course and allowed me to enjoy the IDC so much more. There were no long days in the classroom for me!"*

Tracy

#### The Basics

his subject is much simpler than some instructors make it out to be.

Take a look at the video with an open mind and you’ll be surprised at how easy it is.

#### How it's done

**Metric**In fresh water, it’s often as simple as taking one number (displaced water) from another (weight).

In salt water, first divide the weight by 1.03, and then go ahead as with fresh.

**Imperial**

Multiply the cubic feet of displaced water by 62.4 in fresh water, or 64 in sea water, then deduct that number from the weight of the object. Divide that number by either 62.4 or 64 depending on fresh or sea to get the answer in cubic feet.

Now we move onto the displacement questions. Make sure that you read the question!

Check if it’s in salt or fresh water!

### PADI IDC Exam Questions

Your dive boat bumps into a large box floating just under the surface of a freshwater lake. After bringing it aboard, you determine the weight to be 75 kilograms/165 pounds. How much water does it displace?

75 litres / 2.64 cubic feet

A 200 kilogram / 440 pound anchor that displaces 127 litres / 4.5 cubic feet of water lies on the sea floor in 17 metres/56 feet. What is the minimum amount of water that must be displaced from a lifting device to bring the anchor to the surface?

67 litres / 2.38 cubic feet

If a diver weighing 75 kilograms / 165 pounds in neutrally buoyant in fresh water, the same diver with the same equipment would _____ in salt water.

###### Float

What is the minimum amount of water that must be displaced from a lifting device to bring a 453.5 kilogram / 1000 pound anchor to the surface from 40 metres/ 132 feet of salt water in the anchor displaces 254.8 litres/nine cubic feet?

###### 185.5 litres/6.6 cubic feet

### How did you get on?

Don’t worry if you got a bit confused with these questions. They’ll become clearer with practice.

The more that you practice, the more you’ll understand and the more confident you’ll get.

### Questions on other subjects

## Mock IDC Theory Exam papers

If you enjoyed answering these questions, you could answer a LOT MORE!

Click the link to find a whole load of Replica Mock PADI IDC Exam Papers.

For both Theory and Standards.

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Peter

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