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Illustration of Newton's third law and the Law of Conservation of Linear Momentum

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Source:
Author: Jerry Silver

The Idea

In this experiment, you launch a 2-liter soda bottle into the air. Your fuel is water, which is propelled downward by air pressure forcing the rocket upward. This experiment is a good illustration of Newton's third law and the law of conservation of momentum, and it lends itself to a nice, friendly, competitive "space race."

What You Need

• 2-liter soda bottle (water bottles are not necessarily capable of sustaining internal pressure, as soda bottles are)
• nose cone fabricated from a cardboard party hat or a cone formed from poster board and tape
• cardboard for fins
• glue gun or tape
• water
• hard rubber stopper that just fits the top of the bottle (the stopper should be snug enough to seal the bottle while it is being pressurized, but not oversized to the extent that it prevents the bottle from launching)
• bicycle pump with a one-way valve (or an electric pump or compressor)
• optional: support to serve as a "launch pad" for the bottle (for instance, made from a tripod built from PVC pipe sections). See Figure 27-1.

Build the rocket

1. Slide the open end of the bottle over the vertical rod of a ring stand for easier assembly.
2. Use the glue gun to attach fins to the rocket (remembering that the flat side of the bottle is the top of the rocket). Be careful not to apply excessive heat, which could melt a hole in the bottle.
3. Attach a nose cone to make the bottle more aerodynamic. Use poster board or a coneshaped party hat.
4. Fill the bottle from about one-quarter to one-third full.

Assemble the launcher

You can do this in several ways. If you are planning many launches, you may want to go for something more elaborate. The basic parts are:

1. An air pump or compressor.
2. A one-way valve: The simplest way to do this is to insert a needle (available at any sporting-goods supply store) used to inflate footballs and basketballs through the stopper. With this method, no release mechanism is needed because the rocket will take off as soon as enough pressure builds up to overcome the force holding the stopper in the bottle.
3. A release mechanism: A metal "claw," which holds the bottle in place until the pressure builds to a certain level, allows a greater pressure to build up in the bottle. This can be mounted on a wooden or PVC tripod structure. You can also hold this in your hand, but be prepared to get wet as the "fuel" surges downward from the bottom of the rocket.

Launch the rocket

1. Insert the stopper into the bottle.
2. Secure the bottle onto the launcher. Move the holding mechanism into place (or hold it if that is what you are doing).
3. Pressurize the bottle. The maximum air pressure should not go above 80 to 100 psi (pounds per square inch) to avoid bursting the bottles.
4. Use a string to remotely release the release mechanism.

Expected Results

The rocket will ascend vertically. The upward leg path can take as long as about 4 seconds corresponding to a maximum height of more than 75 meters (over 250 feet).

Why It Works

The air pressure forces the water downward with a high velocity. The mass of the water times the velocity of the water represents the downward momentum of the water. Conservation of momentum requires an equal momentum upward that is applied to the mass of the bottle, which acquires a velocity to take it upward. Another way to say this is the action of the downward force of the water is counterbalanced by an equal and opposite reaction that drives the bottle upward.

Other Things to Try

Bottle rockets can be made more elaborate by adding fins. A parachute, made of the clear plastic used by dry cleaners, can be added to keep the rocket in the air for a longer time or to release a payload consisting of a tennis ball or other object.

You may want to see the Mythbusters episode, where they explore the use of bottle rockets to propel a person. Note, for safety reasons, they confined their efforts to dummies.

The Point

This is another example of conservation of linear momentum and Newton's third law.