Fluid Physics Experiments

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Liquid Layers

An Experiment With Density, Bouyancy and Polar Solvents

What you need:

1) A Small Jar

2) Water

3) A Small Spoon

4) Cooking Oil

5) Food Coloring


What to do:

1) Fill the jar 1/3 with water

2) Stir in a few drops of food coloring to color the water

3) Slowly, fill another third with oil

4) Tightly close the jar and shake

5) Wait and watch for about three minutes

What Happens

When you pour the oil slowly onto the water, it forms a layer on top of the water, because it is less dense than water – that is, the mass of one liter of oil is less than the mass of one liter of water. Thus, the buoyant force on the oil due to the water is greater than its weight and, by Archimedes’s principle, the oil floats – just like a solid object would.

When you shake together the oil and water, they separate quickly, because they are “immiscible” (literally non-mixable). This is because of the differences between the water and oil molecules: Water is a “polar” molecule – meaning its positive and negative charged parts are separated onto opposite ends of the molecule and oil is non-polar. This means that the oil molecules don’t interact with the water molecules and there is no attraction to drive the two fluids to mix.

Generally, non-polar fluids do not mix well with other fluids (even other non-polars), because without the attraction there is no reason for the fluids to mix (or stay mixed).

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Candle in a Jar

Seeing the Weight Of Air

Did you know that air has mass? Because the weight of the atmosphere has been pressing down on you all your life and your body is adapted to it, you don't even notice it! But try traveling to a place at high altitude and you'll quickly notice it's lack.
Even if you are intellectually aware of the mass of air, there are a few experiments like this one (and the "air weighing balance" and Bubble Inertia") that allow you to directly observe the air's mass.

Things you will need:

1) A candle

2) A jar. The jar needs to be tall enough that top of the candle will be well below its lip when the candle stands in the jar. You won't need a lid for the jar, so don't worry if you don't have one.

3) Plasticine to help the candle remain standing when the jar moves.

I found that play-doh had trouble sticking to the glass jar and the candle, making the experiment difficult to set up!

4) Matches or a lighter. Like all experiments using fire, if you're not allowed to play with fire by yourself, ask an adult to help you with this experiment.

What to do:

1) Use the plasticine to stand the candle inside the jar. Make sure it's held securely so that it stays standing when you shake the jar gently from side to side.

2) Light the candle. Either use another candle to reach in and light it or turn the jar upside down.

3) Now, before you go any further, try to guess what the flame will do when you move the jar from side to side.

4) Now that you have your guess, try it out. Move your jar slowly from side to side. Pay close attention to the flame. Which way does the flame move when the jar starts moving? When you stop the jar? If you are not sure why this is interesting, think about which way you move when you are in a car that starts or stops.

What happens?

When you ride in a car that is accelerating, you feel a "force" that pushes you in the opposite direction. When the car speeds up, you're pushed backwards into your seat. When the car turns left, you're pushed to the right. When the car brakes, you're pushed forwards.
These "forces" are not true forces - nothing is actually pushing you forward when the car brakes suddenly. Instead, these "pseudoforces" are due to your inertia and its tendency to travel in straight lines at constant speeds (Newton's First Law).

However, the flame moves differently. When the candle accelerates to the left, rather than trailing to the right the candle flame points to the left! Then, when the jar is moving at a constant velocity the flame points upwards. Finally, as the jar decelerates back to stationary, the flame trails to the right!

We know that it's not just a breeze causing the unusual motion - the walls of the jar protect the flame from the wind.

When everything is stationary, the flame points upwards. This is because the flame heats the air around it. Hot air is lighter than cold air, so the cold air sinks to the bottom of the jar, displacing the warm air. This causes the warm air to rise straight up, so the flame also points upwards.
When the jar is accelerating, the inertia of the air causes it to be pushed to the back of the jar (just like the way you are pressed back into your seat in an accelerating car). Because the cold air is heavier than the light air, it accelerates slower and is pressed back harder against the back of the jar, pushing the hot air slightly forward as well as upwards. Thus, the candle flame points forwards. Similarly, when the jar comes to a stop, the cold air goes to the front, pushing the warm air (and the flame) to the back.

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Will a Grape Float or Sink?

Will a grape float or sink in water? How about salt water? Learn about bouyancy and Archimedes' principle with this fun experiment.

What you will need:

1) Two Clear Glasses

2) Warm water (Hot faucet water will suffice)

3) Grapes

4) Salt

5) spoon

What to do

1) Fill the glasses

2) Disolve about three teaspoons of salt in one glass. Be sure to stir the salt into the water. Using warm water improves the disolution.

3) Place one grape in each glass. Do they sink or float?

What Happens

You should see the grape in the fresh water glass sink to the bottom and the slat water grape float.


Recall Archimedes’ principle: A floating object will displace a volume of fluid that has a weight equal to the weight of the object that is floating.

If the object weighs more than its own volume in fluid, then it will sink.. In other words, a floating object is less dense than the fluid, but an object that is denser than the fluid will sink and the bouyancy force felt by an object is equal to the weight of the fluid displaced.

The grape is denser than fresh water, so it sinks in the fresh water glass.However, when you disolve salt in water, you increase the water's density to a value greater than the density of the typical grape - so the grape floats in salt water.

Moreover, the level the grape floats at is determined by the density of the water - if the water is very dense, only a very small part of the grape needs to be submerged to generate a sufficient bouyancy force to hold up the grape's weight, but if the water is only slightly denser than the grape, it will take most of the grape's volume to generate the required bouyancy force. Thus, you can use the height of the grape to estimate the amount of salt in the water.

Starting with fresh water, sink the grape in the glass. Slowly disolve a small amount of salt in the glass. Slowly increase the amount of salt until the grape rises to the top. Now, as you add more salt, the grape should rise upwards. There is a maximum amount of salt you can disolve into the water, so you might not be able to lift your grape a long way out of the water, but with care, you should be able to observe the grape's upwards motion.

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