For the balloon on the line:
Blow up the balloon and either hold it closed, or clip it with the binder clip to stop air escaping.
Attach the balloon to the straw with tape.
Release the balloon go.
For a simple activity, but one that is hard to control variables, the balloon can just be released into the air.
The force of the air rushing out pushes back on the balloon, making it move. (Newton’s 3rd Law: for every action there is an equal and opposite reaction).
Variables to explore:
1. Ask students to blow the balloon up to different amounts, and compare how far it goes each time.
2. Try adding mass, for example by adding pennies to a pot that is attached to the straw. If students are old enough to blow the balloon up to a consistent diameter/length, the results can be graphed for a visualization of this inverse relationship. For younger students the data is quite messy, I think because the balloons are not consistently blown up, and from other variables, so discussion can be around which number of pennies the balloon went the furthest (generally low numbers of pennies) and the balloon went the least far (generally high numbers of pennies).
The same principal is used to make rockets go into space. Rocket fuel is burned to produce a gas (see molecular modelling of real rockets). The gas produced builds up to enormous pressures and is released out of a small hole called a nozzle (like in your balloon). The gas rushing out of the nozzle from the back of the rocket, pushes the rocket upwards.
For discussion around the added mass, fuel is a significant mass in the weight of a rocket, so makes a large difference in how fast the rocket can be moved. The amount of fuel needed is calculated very precisely to eliminate any extra mass.