Water bottle rockets
Launch a plastic bottle to infinity and beyond!
Learn all about Newton’s third law of motion, with this rip-roaringly raucous rocket launcher!
Please note: This experiment needs to be adult-led, due to the use of the utility knife and the pressure generated by the rocket itself. However, by talking your child through the steps, and by letting them handle the safer parts, you can teach them a valuable lesson, and enjoy a whole afternoon of fun together.
Making the fins of the rocket and connecting the pump
- Use your ruler to mark out the middle points of the foam board. Draw a couple of dots – as this will make it easier to draw a straight, accurate line.
- Now, from the top of your board, draw a line through the points you just marked up, going all the way from the top to the bottom.
- Take a utility knife, and cut down the line you just drew, as steadily as possible. You may need to do this a few times to make sure it cuts right through the foam core.
- Pick up the board, and break it in half along the line you have just cut.
- Next, draw a line from the top corner of your board to the bottom of your board, drawing out the fins of the rocket.
- Cut down this line again, and pick it up and break it in half again. You should now have four fins of foam board.
- Pour 300 – 500ml of water into the bottle.
- Create a hole in the cork. Take the nail and drive into the cork. Be careful as you push, and go slowly, so that the nail doesn’t suddenly push out and injure you. You may need some pliers to pull the nail back out again.
- Thread the needle from the bicycle pump through the hole in the cork so air can be pumped into the bottle. Make sure it goes all the way through.
- Push the cork (attached to the bicycle pump) into the neck of the bottle.
- Turn your bottle upside down.
- Now attach your fins to the bottle. Take your first fin and tape it to the side of the bottle near the neck, making sure that the bottom of the fin touches the ground.
- Take your second fin and tape it on the opposite side of the bottle, directly across from the first fin.
- Repeat this for your third and fourth fin. Make sure that all four fins are equally spaced apart.
Making the ‘nose’ of the rocket
- Take your tennis ball. Place it on the top of your rocket (the bottom of the plastic bottle), and tape it down, until you have a solid cone of tape over the top of your rocket. Attach it by sticking down the tape firmly with your hands.
Launching the rocket
- Congratulations! Your rocket is ready for launch! Go outside with your little one, and look for an open space free from hazards and obstructions, explaining why this matters.
- Place your rocket on the ground so that it is aimed directly upwards.
- Quickly pump the bike pump until your rocket flies away. Three! Two! One! Lift off!
- If your child is unsure of what is happening and why, you can teach them the basics of physical propulsion, using the handy information below.
As we pump air into our water bottle rocket, we’re adding more and more air molecules into the bottle. These air molecules press against the walls of our bottle and generate pressure. The more air molecules we add into the bottle, the higher the pressure inside the bottle gets, and the more potential energy is stored inside our bottle.
Once enough pressure or potential energy has been added to our bottle, the cork is forced from the bottle’s opening, and all of the stored energy and pressure is released, forcing the air and water out of the bottle in the form of thrust.
What happens next in our experiment is a great example of Newton’s Third law of Motion, which states that for every action there is an equal and opposite reaction. When the water and air is released, it causes the action that generates force downward toward the ground. This action is followed by a reaction that propels in the opposite direction.
There are no chemical reactions occurring here; only a mere physical release of air and water. The amount of water, air pressure and the size of the bottle’s opening will change how much force is generated, and influence how far the bottle will fly.
- 30 mins
- Access to a clear open space (ensure there are no nearby obstructions, such as people and buildings), an empty 2 litre plastic bottle, a cork, a tennis ball, some tape, a nail, a foam core board (ideally measuring around 12 x 20 inches), a utility knife, a ruler, a large cutting board, a pencil, a bike pump, a measuring cup, an apron, a lab coat (optional), goggles
- Patience, perseverance, problem solving
- Little physicists