So the mass of the tiny amount of water spewing out of the bottleneck times its velocity is equal to the mass of the rocket (with all that water) times its velocity. Conservation of momentum correlates with the mass and the velocity.

In the beginning, when water just starts to spurt out, that water is pushing against the bottle plus all of the remaining water. When we think about the conservation of momentum in our water rocket scenario we need to recognize that the weight of the rocket is always changing. There is an optimal amount of water to fill your soda bottle rocket with, and that optimal amount isn’t as much as possible. Why does the amount of water in my soda bottle affect the rocket launch height? Seriously, that happened to us…the top of the rocket (aka the bottom of the soda bottle) gave way because we loaded it with too much pressure. Of course, if you add too much pressure the bottle will not be able to hold it and will explode. The high pressure corresponds to a larger force pushing the water out. This is why a larger pressure, or a more pumped up bottle, will fly higher. This force is what will push the water out of the bottle when we pull the trigger and quickly release the pressure. We are pressurizing the air above the water in the soda bottle, which means we are putting a force over the whole surface area of the water in the bottle. Pressure is defined as the force per unit area. We use the bike pump to add pressure to the bottle. Sure the water will naturally want to leave the bottle through the bottleneck since gravity will be pulling it down, but we need something more. We need to create a system where the water wants to escape through the bottleneck, hurling the rocket up, but can’t until we pull the trigger. Our water rocket launcher doesn’t just launch the rockets when you put them on the PVC tube. Why do I need a bike pump with my water rocket launcher? On the contrary, if you were very heavy and decided to throw ping pong balls instead of cannon balls you would move very slowly backward while the ping pong balls flew forward. If the cannon balls are very heavy and you are light as a feather then you would move very quickly backward while the cannonball lazily rolled forward. As you throw these cannonballs to your front – which way do you skate?Īs you put a force on the cannonball to hurl it forward the cannonball puts a force on you to move you in the opposite direction, backward. Imagine you are on an ice rink with superhuman strength carrying 100 cannon balls. This is because of the conservation of momentum. As water pushes down towards the ground it pushes up against the rocket, allowing it to take flight. The rocket moves because of the water pushing out of the bottleneck. During our community science programs, I got a lot of kids saying the bike pump made the rocket fly, or that air made the rocket fly. This is a great question to ask your kids. We will also take a look at finding the answers to the soda bottle rocket challenges that we posted previously. In this post, we will take a look at the physics of what makes a water rocket launch so high into the air. If you did our water rocket launcher activity your kids might be wondering how their water rocket launcher flew so high. Share on Pinterest Share on Facebook Share on Email Learn the basics of what makes a water rocket fly high.

Water bottle rocket