A Blow by Blow about Bernoulli's Law

 

Bernoulli's Law is one concept in understanding how airplanes fly.  In the following lab you'll get a chance to experiment with object and try to invoke this principle.

(By the way, it really isn't the main reason planes fly, though most people think it is.  If you want to be a real aviation smarty pants research the Coanda Effect.)

 

So what do I really need to know to begin?

One way of describing Bernoulli's Law is that the faster a fluid travels over a surface, the less time it has to push on that surface.  So what would you expect to happen?  That surface moves in that direction!

Two key things to remember:

 

Tricks of the Trade:

In the above description we talk about a fluid traveling over a surface.  Most of us, when asked to name a fluid would probably come up with a liquid, like water.  But the official definition of a fluid is "a substance such as a liquid or a gas whose molecules flow freely, so that it has no fixed shape and little resistance to outside stress".

So based on this definition you can see that a fluid could be a liquid or a gas – for instance air.

 

Background Experiment:

Try this before you begin.  Hold a sheet of paper against your bottom lip and blow.  If the air leaving your lips is fast enough, the air pressure is reduced.  When the air pressure is reduced, it doesn't push down on the surface of the paper as much, so the paper appears to "fly" upwards.  But, check it outŠthere isn't any upwards force, is there?  No one is pushing the paper up. 

That is an example of Bernoulli's Law.

 

The Challenge:

Using the provided materials see if you can figure out how to apply Bernoulli's Law in each situation.

                     

Directions:

Kissing Balloons

  1. Hold the straw between the balloons horizontally.
  2. Level the balloons so that they hang at the same height.
  3. Without touching the balloons, find a way to apply Bernoulli's Law so that the balloons touch or "kiss".

 

Floating Golf Ball

  1. Place a golf ball in a cup and take it to the sink.
  2. Find a way to make the golf ball float using Bernoulli's Law and water from the faucet.

 

Ping Pong Ball Launch

  1. Locate the labeled paper towel roll with one end taped off and a ping pong ball.
  2. Place the ping pong ball in the roll and hold it vertically with the taped end pointing down.
  3. By applying Bernoulli's Law, can you make the ping pong ball "launch" out of the tube?

 

Super Soda

  1. Place several drinking straws on the table.  (Hint: Spread them apart, this won't work if the straws are too close together.)
  2. Put two empty soda cans on the straws, about 1" apart.
  3. Using Bernoulli's Law, can you make the cans move together?

 

Collapsing Tent

  1. Fold a sheet of 8.5" by 11" paper in half.
  2. Set it up like a tent on the table.
  3. Using Bernoulli's Law, make the tent collapse.

 

Questions:

Now that you've done several experiments, you should have an idea of how Bernoulli's Law affects motion.

  1. For each of the experiments, write a paragraph description how Bernoulli's Law works in that situation.
  2. Additionally draw a sketch and label the area of low pressure and the resulting motion for each experiment.

 

Extension:

Design your own demonstration that illustrates Bernoulli's Law.  You should include a list of materials and clear instructions so that another person could repeat your demonstration.  Additionally include a sketch of the situation and label the area of low pressure and the direction of the resulting motion.