My Sub Sinks!

The Challenge:

To build a working mini-sub that will move as fast as possible at constant depth .

But to do this you will need to learn some Physics.

Question:

Why does something sink in water?

Because the forces acting on it are unbalanced.

Question:

What are the forces acting on a submarine?

So what is happening to the sub in these diagrams?

 

So the sub moves in the direction of the biggest force?

No. It accelerates in the direction of the biggest force!

It is important to understand that acceleration (change in speed) happens whenever there is an unbalanced force (or resultant force).

Constant speed only happens when the forces are balanced (or resultant force is zero).

Video:

Air forces for a parachutist and speed time graph

Questions:

Worksheet on yer bike

 

So... how do we stop a sub sinking?

Now try this question

Homework:

Draw a series of force diagrams showing an imaginary bicycle journey to school. This should include a hill or two, some constant speed and might even include a crash or traffic jam!

Ref:

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My Sub Won't Sink!

Why do things sink?

So why do things float?

 

Archimedes Principle...

The buoyancy force is equal to the weight of the displaced fluid.

 

We make things float by making them lighter than the water they dispalce.

 

We need to understand density.

 

We will find the density of 5 different materials by measuring their masses using a top pan balance, and their volume using a measuring cyclinder to determine their volume.

Finding volume by displacement method...

Task:

Explain how a Cartesian diver works, and do the question on cartesian divers

Question:

What have we determined about the weight of the submarine we will make?

 

This is called NEUTRAL BUOYANCY.

Ref:

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Neutral Buoyancy

Question: If we put a ping-pong ball in water does it float or sink?

Float!

Question: So what must we do to it to make it neutrally buoyant?

Add weight.

Question: But how much?

 

Aim: You must find how much mass is needed to make the ping-pong ball neutrally buoyant.

Apparatus:

 

Task:

Complete the experiment, and produce a lab report for the experiment with:

• Prediction
• Diagram
• Method
• Table of results
• Conclusion

Homework: Complete the write up.

Ref

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Building a Simple Sub

Real Subs have to be able to change their buoyancy so they can ascend and descend.

Flash animation to illustrate this at www.howstuffworks.com

Experiment:

Complete this experiment to show how a real sub changes it's buoyancy.

 

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Moments

So you have a 2 litre drinks bottle that you need turn into a neutrally buoyant submarine that moves as fast as possible through the water at a constant depth.

How much ballast do you need?

How will you distribute it to make sure the sub stays level?

 

If the weight distribution of the sub isn't right, then it will try to go up or down.

This turning effect is called a moment. To keep the sub level do this you must consider the moments acting on it.

Video: Moments

To make sure our subs don't have any turning effects, we need to understand and use the law of moments...

Demonstrate this with the applet

Task:

Now try the practical exercise and questions.

 

Challenge: Can you lift the oblisk?

Ref:

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Getting the Balance Right

We must arrange the ballast in our sub, so the moments in the clockwise and anticlockwise directions are the same.

 

1. Dismantle your sub, and find it's balance point (Centre of Mass).

2. Collect the correct amount of balast (from your calculation).

3. Attach the balast into your sub at the correct location?

4. Seal the sub such so that it is water tight (why is this important?), and the ballast is on the inside.

5. Test that it is neutrally buoyant and floats level in the water.

You won't be able to concentrate all the balast in the exact centre of mass, think...

Hint...

 

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Testing the Sub

Time to see if all the hard work paid off...

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Energy

Energy is a fundamentally important consideration in Physics, and in Submarine design.

What are the 9 different sorts of energy?

K inetic

E lastic Potential

S ound

C hemical

H eat

G ravitational Potential

L ight

E lectric

N uclear

In a submarine there must be enough energy to not only power the sub's movement, but also to heat it (deep oceans get very cold), guide it (navigation computers are very power hungry), sustain the crew (they need oxygen- there's not much of that underwater!).

This means that Submarines need to carry a lot of stored energy. How can they do this?

Many submarines are now nuclear powered because it allows them to generate the required power with very little fuel.

Question:

Using Einstein's famous equation E=mc², work out how much energy a submarine's nuclear reactor could generate from turning 1kg of matter into energy.

Hint: You need to know the speed of light, c = 3 x 10⁸ m/s

Question:

Is nuclear power the best way to power a submarine? What are the other options?

 

Task:

Draw 3 sankey diagrams for energy changes that happen onboard a submarine. For each one say which system it represents.

eg: The Motor System

 

Task: Watch the MegaStructures: SuperSub video, and answer these questions.

 

 

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