Warning: This experiment can be dangerous. Do not shoot your rocket at animals or people. Only use your rocket in large open spaces.
What you need
stick of lip balm
butter knife
plate
What to do
Take the cap off the lip balm container.
Wind the stick of lip balm out as far as it will go.
Pull the lip balm stick out of the container.
Cut the lip balm off, as close to the plastic piece as possible, scraping out any remaining lip balm. Put the lip balm aside in a small container for use – don't waste it!
Pull the twistable base out of the lip balm container. This is the bit you normally wind up and down.
Gently reinsert the base into the container, but make sure it does not click back into the bottom of the container.
Reattach the plastic piece left over from the lip balm to the stick coming out of the base.
Put the cap back on the container.
Take your rocket to a wide open space away from other people. Make sure the cap is facing away from your body. Use your palm to push the stick in quickly. What happens?
Reclaim the lid and put it back on the container. Try the experiment again, pushing the stick in slowly. What happens this time?
What's happening?
To understand what's going on with the rocket, you'll need to know a bit about Newton's laws of motion.
The first law says: an object will remain at rest, or continue to move at a constant velocity, unless a force acts on it.
Newton's laws are never as complex as they first appear. Let's break it down, starting with what a force is and working out what forces are acting on our rocket.
A force in this experiment is anything that can change the speed of the rocket (the cap) as it travels in a given direction.
A model rocket, like our pocket rocket, is subjected to four different forces in flight; thrust, weight, lift and drag.
Thrust is the force that starts our rocket flying. You can apply thrust by pushing in the base very quickly. The rocket will remain at rest until you apply this force.
By squeezing air into the lid quickly, you are increasing the air pressure inside the lid. Force which was once distributed over a large area is suddenly concentrated in a small area and the lid flies off.
Newton's third law of motion explains why the lid goes flying off – for every action there is an equal and opposite reaction. The air inside the lid escapes out the back, which propels the lid forward. These are equal and opposite reactions.
If you push the base in too slowly, air has time to escape out the sides so the air pressure does not increase much. Friction (another force) between the lid and the container can also stop the lid from flying off in this case.
When thrust becomes greater than drag, acceleration forward will occur. Yet when drag becomes greater than thrust, the rocket will decelerate (move slower over time).
Your rocket starts off with great thrust and perhaps some lift, and comes to land because drag and weight kick in. This fine balancing act is what Newton's second law of motion is all about - force on an object is equal to its mass (the amount of matter or stuff it's made of) multiplied by its acceleration.
Applications
All flying objects from planes to birds are subject to lift, weight, drag and thrust.
The flight of a compressed air rocket, like the pocket rocket, is very similar to the flight of ballistic shells and bullets. This is because in both cases the thrust is completely spent at the start of the projectile's flight.
This is different to full scale rockets where thrust force is applied to the rocket for a large portion of the flight while the rocket fuel burns.
Try some other Science by Email activities using rockets:
