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2. Crash Bang! - Exciting experiments for you to try at home...
Floating and Sinking
What you need
• A few unopened cans of fizzy drinks, diet and regular varieties.
• A bucket or basin of water.
• Some salt.
What to do
1. Pop your cans in the bucket of water and watch which ones sink and which ones float.
2. Are you finding that the diet drinks float while the regular drinks sink?
3. Find one which sinks (or at least bobs along the bottom), and keep it in the bucket - take the others out.
4. Now add salt reasonably liberally to the water in the basin and see if the can begins to float.
What's happening?
When an object is dropped in water it displaces exactly the same volume of water as it occupies. The water wants to sink back down to its lowest level, as everything does when acted on by gravity. To get to its lowest level the water needs to push the object out of the way, creating a force pushing the object up and out of the water. How successful the water is at pushing the object up depends entirely on how much the object is being pulled down by gravity, because it too is trying to reach its lowest point.
The force pushing the object out of the water is exactly equal to the weight of the displaced water. The force pulling the object down is the weight of the object. Whichever weighs more wins: if the water weighs more the object floats, if the object weighs more it sinks.
To know whether something will sink or float without getting it wet requires knowing the density of the object. The density is the amount of material squeezed into a certain volume. Since both the displaced water and the object occupy the same volume, comparing the density of water and the object will tell us which weighs more. Wood is less dense than water so it floats; iron is more dense so it sinks.
When there is a composite object like an enormous ship we can't just look at one material. A ship floats because the weight of water displaced is greater than the weight of the ship. The total density of the ship is less than water - this is because the ship is filled almost entirely with air.
When using fizzy drinks this experiment does not always tell you which is the sugary drink and which is the diet drink. Instead it will only tell you which can is more dense. More often than not the sugary (non diet) drink will be more dense; there is simply more stuff dissolved in the drink, but it will depend on the recipe.
Adding salt makes the water weigh more, so it pushes the can out with a greater force and suddenly the sinking can begins to float.
This activity came from the Planet Scicast site
There are some fantastic films on there - why don’t you have a go and make one yourself?
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November 25 is the start of 
