NYU Physics Demos

Spinning Torpedo

Topics: Mechanics - Acceleration, Conservation of Momentum

Equipment: Torpedo mounted to rods on a stand, CO2 canisters, needle punch, clamp.

Setup: Use a large clamp to secure the base to a table. The rocket will move very quickly, and if it is not fastened down it will become unstable and fall over. Insert a CO2 canister into the rocket. Use the needle punch to break a hole in the end, releasing the pressurized gas inside the canister.

The gas inside the canister is compressed, so the molecules have a certain amount of potential energy. Once the canister is punctured, this energy is converted to kinetic energy, and the molecules shoot out of the canister. The energy is not conserved, because the ejected molecules “lose” their kinetic energy to external resistive forces. Momentum, however, is always conserved. When a single CO2 molecule launches out of the canister, it accelerates (experiencing a force, F=ma) and exerts a force on the torpedo, in the opposite direction (Newton’s Third Law). The force from a single molecule is negligible, but there are many, many molecules in the canister. The sum of the forces from all the molecules causes the torpedo to accelerate, and being mounted to a ball bearing, the torpedo will spin around a central axis.

Location: Shelf B-f6

Notes: The canister will be very cold after it is emptied. Discard after using it. There are more canisters in B22.

Keywords: Force, Velocity, Mass, Conserve, Fly, Rotate, Momentum

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