You've probably seen a balloon go whizzing around in the air before, but why does this happen? You might say that it's because the air from the balloon pushes it along––and you wouldn't be wrong! But why don't we go whizzing around the room when we, say, blow out of our mouths?
The answer can be found by investigating thrust, a force that occurs when an object (i.e. a Space Shuttle, balloon, or person) expels or accelerates mass in a specific direction. This might sound complicated at first, but we can get the hang of it by thinking about our balloon example:
In this experiment, we're going to take a closer look at thrust and forces and see what happens when we use our knowledge of thrust to our advantage.
As in the picture, tape the straw on the balloon so that the straw is facing one end towards the balloon's nozzle and the other towards the opposite side of the balloon. Don't blow up the balloon yet, that part's coming soon!
Thread the string through the hole in the straw.
Tie both ends of the string to two different tall objects, far enough apart so that there're a few meters of distance in between them. We used two chairs. The string should be as taut as possible.
Bring the balloon rocket closer along the string to the object that's facing its nozzle. When you're ready to launch, blow up your balloon as far as you can without it popping and hold the nozzle with your fingers.
Launch your balloon rocket by releasing the nozzle. The balloon should zoom along the string as air shoots out.
Now, repeat steps 4-5, but this time, try different amounts of air. See if blowing up the balloon about half as much shoots the balloon half as far. Or maybe see how much air the balloon needs for it to move at all.