Objective: Preparing you for your IIA next

week by getting you to think about

experimental techniques.

ROCKET SCIENCEMiss Amlani

Levitating ping-pong balls

Turn the hair dryer on high and point it toward the ceiling. Make sure to use a cool-air setting!

What do you think will happen if you gently place the ping pong ball into the stream of air? Will it blow away, drop to the floor or float? Try it and see.

Key word: PREDICTION

Levitating ping-pong balls

The air leaving the straw blows the ping-pong ball upwards, but what keeps it there and why doesn’t it fly off and fall to the floor?

Key point: EXPLAIN your findings using Scientific knowledge.

Why does it work?

The effect has been described in the 1930s by Coanda a Romanian aeroplane designer. He showed that when air, or a fluid, flows over a curved surface, it can stick to the surface and follow it, so the flow also becomes curved. This means that when the ping-pong ball sits in the centre of the air-stream, the flow traps the ball by passing around it on all sides, sticking to its surface.

It remains in one place because of Newton’s Third Law (for every action there must be an equal and opposite reaction.) If the ball tries to move in any direction, the air sticking to its surface will be pulled with it. But if the air is being moved, then there must also be a force pushing back on the ball in the opposite direction, which is what holds it steady. This is why it bobs about in one place.

How could we take this experiment further?

See what happens when you:- Try tubes that are longer or shorter or wider or skinnier.- Try to float objects in the air stream. - Try to float two or more balls in the same air stream. How many can you float at once? How do they behave when there is more than one?

How does this apply in the real world?

http://www.youtube.com/watch#!v=YRrzcnBPpm4&feature=fvw

How can we use this to explain how the wing of an aeroplane generates lift?

Liquid behaving badly

Is it a solid or is it a liquid? Make a really strange mixture and find out how it behaves.

Liquid behaving badly

http://www.youtube.com/watch#!v=BN2D5y-AxIY&feature=related

How does this work?

Cornflour is made up of lots of tiny (<0.01mm) starch particles, these are very attracted to water so the water gets in amongst them very quickly.

The water acts as a lubricant, so when you move it slowly the particles have time to move past each other and they can flow like a liquid.

However if you apply a rapid force it causes the particles to move slightly causing the particles that are almost touching to jam together. and the water that was between them moves sideways slightly into the gaps.

Now instead of having lots of lubricated individual particles you have a solid structure of lumps touching each other which can't flow

How does this apply in the real world?

http://www.youtube.com/watch?v=qfhw6I_uBQg

Cola Volcano

You may have heard about the Mentos and Coke experiment, find out how to make an erruption with a drinks bottle, and what it has to do with volcanoes.

How does it work?

A drink is fizzy, because it has lots of carbon dioxide ( CO2 ) dissolved into it under pressure, while the pressure is maintained the CO2 stays in solution. When you open the bottle the pressure is released, and the CO2 starts to come out of solution.

The bubbles need something to start to form on though and because the inside of the drinks bottle is very clean there are very few places where they can form, so it will take a couple of hours for the CO2 to be released.

The mints are really rough, so when you add them, there are lots of places for the bubbles to form (if you added salt, sugar, sand etc it would work too) so loads of bubbles are created all at once. This makes the liquid hugely bigger forcing it to escape rapidly out out the top, in an erruption.

More about the bubbles

Surface tension is the force that pulls water together into drops, and also is trying to compress bubbles. Whatever the size of the bubble the suface tension is the same.A small bubble has a very curved surface which means that much of this force is acting inwards compressing the gas inside.A larger bubble is much less sharply curved so less of the force is acting inwards so the pressure is lower.

If a bubble gets small enough the CO2 inside is at such a high pressure that it will dissolve again, causing the bubble to collapse.

This means that there is a critical size of bubble, bubbles bigger than this grow and bubbles smaller shrink. So normally no bubbles can form, however if you drop a mint in water the rough surface will make it easier for bubbles to form, as water doesn't stick to sugar as well as other water, and because it will drag little air bubbles along with it trapped in amongst the rough surface.

How does this apply to the real world?

There are many types of volcano, but the ones that errupt explosively, have lots of gas dissolved in the magma. If the pressure builds up and then is suddenly released, these gasses form bubbles, and expand to thousands of times their original volume throwing, rocks, ash and pummice kilometers.

VariablesThe variable which is chosen for an investigationby the person doing the experiments is called the independent variable.

The thing being affected is called the dependent variable.

The factors you keep the same in your experiments (fair test) are called control variables.

The independent variable is plotted on the x-axis of a graph.

The dependent variable is plotted on the y-axis.

Your Turn

Design an investigation using any of the experiments you have seen today in small groups (of no more than 3).

What are you investigating?

What will you be measuring?

What is your prediction?

What will your method be?

What are your variables?