Rocket Tricycle, 1N22.10

Topic and Concept:

• Linear Momentum, 1N22. Rockets

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Location:

• Floor Item: ME, East Wall

Abstract:

Dual-mounted CO₂ fire extinguishers are on the back of a tricycle.

Important Setup Notes:

• Make sure you have plenty of room (at least 5 ft) to stop before firing the rockets!

• If performing the demonstration in a smaller lecture hall, using only ONE CO₂ fire extinguisher is advisable.

Setup and Procedure:

1. Make sure the tricycle's three tires are filled to the specified air pressure.

2. Make sure there are(is) (a)filled CO₂ fire extinguishers on the back basket of the tricycle. If replacements are necessary, switch out the old fire extinguishers by loosing the black knobs which keep the main holding bracket in place. After switching the new extinguishers in to the mount, the holding bracket needs to be tightened again.

3. Be sure the release handles on the extinguishers are situated beneath the push-down lever such that the nozzles are directed away from the rider.
4. Situate the tricycle so that it has at least 50 ft for cruising distance (25 ft if using one extinguisher).
5. When ready, push down the lever from the right-hand side to engage propulsion.
6. A few trial runs are recommended.

Cautions, Warnings, or Safety Concerns:

• Don't make any sharp turns while riding the rocket tricycle, it is designed to assume a roughly linear path. Large deviations from this will result in toppling over (or running into some students).

• Make sure that there is nothing behind the tricycle. The CO₂ cylinder's exhaust is very strong and very Cooled.

• Make sure that you have plenty of room behind the tricycle for the CO₂ cylinders to fire.

Discussion:

From the viewpoint of Newton's third law, when a body exerts a force on another body, that body then exerts a force equal in magnitude but opposite in direction on initial body. Applied to our rocket-powered tricycle, the CO₂ gas is pushed out of the extinguishers by a pressure differential, The force on the gas molecules originates within the extinguishers which are attached to the tricycle. The gas then pushes back on the extinguishers causing the tricycle to move forward.

From the viewpoint of the conservation of linear momentum, our system is initially at rest (p=0). When the CO₂ gas is shot out from the extinguishers, the constituent molecules carry with them a momentum in the reverse direction (p_CO2 = m_CO2 * v_CO2). To conserve momentum, the tricycle must gain the same amount of momentum in the forward direction. While the mass of the CO₂ ejected is relatively tiny compared to that of the tricycle plus the rider, this average speed of the molecules is very high compared to that which the tricycle will gain.

Videos:

• Lecture Demonstration's Youtube Channel

References:

• The Physics of Rockets - Wikipedia

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