The SFK Project: #1 Backstroke? What’s that?

What you need:

• 10 to 20 meter of thin (washing) line (must fit well through a straw)
• Thick straw (ca. 6 mm diameter)
• Rocket balloon (the longer, the better)
• Sticky tape
• Scissors
• Silikon or Teflon spray

What you will get at the end:

But first what in heaven is a “backstroke”?

We will use this funny backstroke in our first experiment as a force to drive a rocket! Yes, in reality, all missiles and all jet planes are powered by this “backstroke force”.

But let’s experiment first. Let’s do this together in a group because we will need more than two hands for our experiment and you probably have only two, right? And maybe we best do this outside getting a little fresh air. But if you must stay inside, look for the largest room or corridor.

How to build:

One of us is a good climber, right? He gets the end of a washing line and climbs up some tree or some monkey bars. When he’s up there, he needs to fasten the end of the line securely and may come back down. I'm too old for climbing. So I tied my rope to the balcony (see video below). If you must stay inside, try to tie one end as high as possible. 

Meantime we have cut off 3 to 4 one-inch pieces of the straw and threaded it on the line. One who has big lungs may blow up the balloon and hold close the end, so the air keeps inside until we give the “lift-off” command (DO NOT KNOT THE END!).

One or best two others of our group take the sticky tape and tear off stripes. Using these stripes, we fix the balloon securely to the straws – just like in the picture. Watch it! The nose of the balloon must point towards the tree or monkey bar. The nozzle where the air gets inside needs to point toward the opposite direction! If you mix this up our rocket will not lift up but instead, dig into the ground.

Now we start the „Count Down“:

10 – 9 – 8 – 7 – 6 – 5 – 4 – 3 – 2 – 1 – LIFT-OFF! This is the command for our “big lung” to let lose the end of the balloon… shshshshsh… off the rocket goes.

Do you want to know why the rocket went up the line, right to the top? So let’s see…

How it works:

To understand our rocket drive, I must explain something else first. It’s one of those essential laws of nature:

Imagine Tom is smashing his fist on a hard table using a high force. This force which does act upon the table is the same force which acts upon Tom’s fist. Tom will painfully feel this force. And this exactly is our law of nature: Everything acting with force upon something will feel the same force acted upon itself but in the opposite direction. This force acting in the opposite direction (you could say which is “coming back”) is called “counterforce”.

Okay, that still was easy enough to understand, right? But now it’s going to be a little more complicated: If you want to move something, you will always need a force to do so. This force is called “accelerating force“. So that’s a force to get something in motion, to “accelerate” it. This could be your bicycle. Your leg muscles would have to bring up this force to accelerate (that means to speed up) your bike. Without any force, everything stays where it is or lays or swims or floats. But where comes the force which speeded up our balloon rocket?

That was such a “counterforce” we’ve talked about earlier: The balloon has pressed air outside. That’s again a task which needs a force to be done. The force comes from its elastic skin which expanded when we were blowing up the balloon. So the balloon skin uses a force to act upon the air to get it in motion and streaming out of the nozzle of the balloon. Yes, sure! Also, the air stays where it is if no force acts upon it – just like every other object does. The balloon produces this force to act upon the air and accelerates it to blow backwards outside at the back of the balloon. And there is of course – according to our law of nature – a counterforce to this accelerating force. This counterforce does act in the opposite direction, which is forward toward the tree or monkey bars. This counterforce is called “backstroke force” and moves our rocket forward on the line. Phew! Wasn’t too complicated, right?

By the way:

Now, if you really want to boast you may ask your parents if they know the „third Newton’s axiom“. Newton is the name of a British scientist. And “axiom“ is simply the Latin word for a basic principle. Our experiment has taught us exactly this third basic principle: „force is equal to counterforce“. Or if you like to boast with some Latin: „Actio = Reactio“. And then simply show them your rocket…

By the way: Our rocket would fly right up to the moon if there would be only the accelerating force of the backstroke. But there are three other forces which slow it down again, preventing the rocket from flying up to the moon:

The air in front of the rocket acts like a “soft wall” which has to be pushed away by the rocket. This is called “air drag”. Ask your parents if you may hold your hand outside the window when sitting in your car driving on the motorway. You will feel this force which will try to push your hand backwards trying to slow it down. Racing cyclists duck down to lower their “air resistance” and the amount of air they must push out of the way.

There is another force which is slowing down our rocket. It acts between straw and washing line and is called “friction”. The straw rubs against the washing line when moving forward. And this slows down the speed of the rocket. The amount of friction force depends on how smooth the surfaces of the two objects are which rub one against the other. Try to ride your bicycle not upon the smooth surface of a road but on rough sand, and you’ll feel what I’m talking about. Can we smoothen the surface of our washing line? Try some Silicon or Teflon spray. You’ll find out the rocket lifts off much faster and climbs up even higher because the friction force is less!

The last force which is slowing down the rocket even makes it sliding back the rope as soon as the air is out (you can observe that in our video). It is also the strongest force acting against our rocket: This force is called "gravitational force" or "gravity". This force is acting on all objects and subjects here on earth. Even on you right now. It makes you stay on the ground instead of lifting off to the moon. It pulls everything and everybody toward the ground. But this gravity force gets smaller and smaller the further away you get from the earth. Have you ever seen pictures or videos of astronauts in the International Space Station (ISS)? They are levitating in the air like helium balloons. On the ISS, there is nearly no gravity force because it is too far away from the earth.

Where I’ve got the idea from:

“The Magic of Science“by Tony Griffith, seminar booklet.

What's coming up:

Next week in SFK #2, we will build a little hovercraft using a CD, a balloon and a bottle cap