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Whelmer #1: Air Cannon

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Illustration of air cannon (a bucket with a hole in the bottom and a sheet of rubber stretched over the top) emitting a ball of air when struck.Description:
This device shoots a "cannon ball" of air up to 100 feet.

Science process skills:

  1. analyzing
  2. predicting
  3. estimating
  4. observation
  5. identifying and controlling variables
Complex reasoning strategies:
  1. induction
Standards:
K-4:
  • Employ simple equipment and tools to gather data and extend the senses (Standard A.1.3).
5-8:
  • If more than one force acts on an object along a straight line, then the forces will reinforce or cancel one another, depending on their direction and magnitude. Unbalanced forces will cause changes in the speed or direction of an object's motion (Standard B.2.3).
  • Energy is a property of many substances and is associated with heat, light, electricity, mechanical motion, sound, nuclei, and the nature of a chemical. Energy is transferred in many ways (Standard B.3.1).
9-12:
  • Objects change their motion only when a net force is applied (Standard B.4.1).
  • Waves have energy and can transfer energy when they interact with matter (Standard B.6.1).
Above Standards from the National Science Education Standards.

Content topics:
  1. fluid dynamics
  2. compression of gases
You will need:
  1. 5 gallon plastic bucket or small trash can
  2. rubber sheet (shower curtain, tarp, etc.)
  3. large hose clamp or string and duct tape
  4. saw or cutting blade
  5. dowel or drum beater
Instructions:
Cut a 2.5 to 3 inch circular hole in the center of the bottom of the bucket. Stretch a sheet of thick rubber or shower curtain over the open end of the bucket. Secure and seal it with a long hose clamp or use string and duct tape. Strike the "drum" head with a dowel or drum beater. A puff of air will shoot out of the hole in the bottom of the bucket, traveling the length of a classroom.

Presentation:
Ask a student to look in the bucket and describe the contents. (Most students will make a quick response and say "There's nothing in the bucket.") Repeat the inquiry, asking the student to think before responding. Elicit the response that the bucket is full of air. Remind students that scientists pause and think before responding to questions.

Traditionally, air cannons have been used to extinguish candles from across the room, and blow paper cups off the heads of students á la William Tell. Ask students to share where they have seen or experienced any device or phenomena similar to the air cannon (nozzles on garden hoses, bellows, water gun, spitting a watermelon seed, etc.; any situation where pressure is applied to a liquid or gas being forced through a constriction).

Content:
This activity demonstrates the fact that air occupies space. As the rubber sheet is pushed into the interior of the bucket, the volume decreases and pressure increases. The increase in pressure forces some of the air out of the hole. The velocity at which the air leaves the bucket is inversely proportional to the diameter of the hole; the smaller the hole the greater the velocity of the air. Students may have experienced a similar phenomenon as they constrict a garden hose to increase the velocity of the flowing water.

The proper name for the air cannon device is vortex generator. The "ball" of air that shoots out of the cannon is actually a flat vortex of air, similar to rings of smoke blown by a talented cigar smoker.

A vortex is generated because the air exiting the bucket at the center of the hole is traveling faster than the air exiting around the edge of the hole. That swirling or vortex motion can be observed if a little smoke is blown into the bucket just before giving the rubber membrane a gentle push.

Assessment:
Type: small group.
Content/Process: Compression of gases (velocity is inversely proportional to diameter of the nozzle or hole through which a gas is compressed).
Age/Level: all.

Factors to be assessed are:
  • Success of the cannon in extinguishing the candle.
  • Identification of the variables affecting the distance the air is traveling.
Allow the students to examine a garden hose and nozzle, a bellows, a cake decorating tube and points, or other devices that constrict the flow of air, water, or other substances. Ask them to describe how they work. Give students the same equipment used to construct the air cannon and challenge them to make a cannon that can blow out the flame of a candle that is placed a set distance from the position of the air cannon. They can construct as many cannons as they wish to devise the most effective one. They should describe the variables that make this cannon successful.

Notes: