Science Olympics 2005: Winter 2005
Water Rockets
Alex Tung and Ross Venook
This event is not unlike the ‘rocket cars’ event of yore. The difference is that these babies will fly in three (3) dimensions. Another key difference is that we are using an air pump—instead of a chemical reaction—to build pressure within the plastic soda bottles.
Basic Event Structure:
Teams will have two attempts to launch their rockets. We will measure final stopping distance by marking with flags in the grass and measuring from the launch point with a tape measure. Furthest rocket wins.
Tutors or students will put the water and plug into each bottle before launch. The tutors will then assemble the launch pad, set up the rocket, and pump to 20 psi using a bicycle pump. Finally, the tutor will pull the nail to release the rocket. If the rocket does not go off within 3 seconds, a tutor may nudge the rocket. If the rocket goes off, its distance will count. If the tutor decides the rocket is a dud, the team will get another chance to launch (no penalty). This includes those rockets which launch on a delay but are clearly suffering from lost pressure (judgment call).
Rules Summary:
-Pressure is fixed at 20psi
-Angle is fixed at approximately 45 degrees
-Teams have two attempts to launch their rockets
-Teams may alter their rockets in any way they wish (add fins, etc.), and they may adjust the water level.
-Tutors have discretion for relaunching false starts/duds
-The rocket that travels the farthest linear distance wins!
Materials:
Each team will get one plastic soda bottle (20 oz., empty).
Everyone will have access to different materials (paper, foam board, tape, markers, etc…) with which to optimize his/her team’s bottle for flight.
The event requires launchers, which the students will not be building. There will be as many launchers as we can put together to facilitate massive amounts of testing (or as many as we have bike pumps). These launchers consist of a wooden base, a #3 rubber stopper with a bicycle inner tube valve through its hole, a number of washers, a few L-braces, and a large nail on a string. The base should be angled some amount (45 degrees?), and the air valve needs to poke through a hole in the base with enough room underneath to attach a bike pump.
Materials Summary:
Rockets
-20oz soda bottle
-materials for fins
- plastic cups
- foam board
- notecards
-materials for cones
- construction paper
- manila folders
-binding materials
- scotch tape
- masking tape
Launcher
-rubber stoppers with hole in the middle
-presta or Schrader bike valves
-bike pump
-launch mechanism to hold pressure and release
Lesson:
The learning objectives for the Water Rocket project are three:
1)Newton’s third law—equal and opposite forces.
2)Pressure—what it is, why it is so cool.
3)Experimental discovery of a Goldilocks Phenomenon (finding the water level that is “just right”)
Newton’s Third Law: For every action, there is an equal and opposite reaction.
We want the students to understand that in order for something to ‘push’ there must be something as strong against which to push. There are a few good/quick examples for this:
Examples of pushing: Skateboard
-Person on a skateboard pushes off of a wall and rolls along.
-Person on a skateboard pushes off of another person (who stands firmly) and rolls along.
-Person on a skateboard pushes off of a limp person—no rolling along because there is no opposite force.
Example of pushing: Walking on ground vs. walking on mud/ice/banana peel/slippery floor (puppy)
-When we step, we are pushing back on the ground. The opposite force from the ground pushes us forward.
-However, when we step on a banana peel, or ice, or mud, the ground isn’t able to supply the equal and opposite force. Therefore, we don’t get the force we need and we just slip (don’t move forward or backward).
Example of pushing: Rocket propulsion (space shuttle, fireworks, etc…)
-The fire and materials coming out the back of the rocket is pushing off of the rocket and vice versa, so the rocket is propelled forward.
Segue: Now…if we want to make a rocket, we need to have some fuel, or some energy, or some way to generate a force behind the rocket so that it can go.
-Where can we get that force? (rocket fuel, explosions, big air blower, etc…)
-How do we generate force with our bodies? (muscles) What kinds of forces do we make? (this is fishing a bit…but we’re looking for anything related to pressure. ‘we press on things’ ‘we blow out air from our mouths’ perhaps someone will sneeze or cough to push this along)
Pressure: Pressure is force per unit area. We will not be sharing this with the students as it is likely confusing in the case of gas molecules exerting force on the walls. Instead we will focus on manifestations of pressure. There are very exciting manifestations.
Examples of pressure: water bottle with air in the top
-Squeeze the bottle without the lid on until the water reaches the top. Easy. You are able to push the air out of the bottle easily because it has the same pressure as all the other air outside.
-Now squeeze the bottle with the top closed. Can you squeeze it all the way? Why not? The water squeezes the air, but since the air cannot escape it builds up pressure – that pressure exerts a force outward on the bottle that you can feel working against your fingers.
-How is this experiment like the skateboard experiment? (air pushing against the cap is hard and pushes back on me).
-We can all try this by breathing out. Then by keeping out mouths and noses closed and breathing out. You build up pressure in your mouth/cheeks. The more pressure there is, the better it is at pushing back on you.
Examples of pressure: balloons
-Last year we made balloon cars. Remember how you could store energy in the form of pressure inside a balloon? When you opened the balloon, the air pressure could be used to propel your car forward: air going back, car going forward.
-Blow up a balloon and let it go.
Recap on Rockets
So: we’ve talked about equal and opposite forces, and we’ve talked about pressure. For the rockets, we will be putting pressure into a bottle and then releasing it. We will let you experiment with a few things. For example, who thinks the rocket will go further if we put some water in it first? Who thinks it will be too heavy and won’t work at all? Let’s go try. After we demonstrate the rockets launching, you will get to experiment with your own rockets. We have materials for changing the shape of your rocket if you want, and we’ll have plenty of time to test things out and then decorate the rockets.
For tutors working with students: Varying/testing the water rockets:
-Water level – vary the water level to find the optimum for distance
-Fins – Fins can help stabilize the rocket so that it flies straighter – but they have to be light!! For tutors: ideally the fins shift the center of drag backwards along the rocket toward mouth of the bottle. It might be hard to construct effective fins on these rockets, especially with the kind of launcher we have.
-Cone – A cone can also help stabilize the rocket and reduce drag/air resistance. Same weight issues apply, plus if the cone’s not straight, there may be problems. It’s possible that a very light cone without fins could be a winner.