Thames & Kosmos AIR Plus Water Power Experiment Manual page 29

EXPERIMENT:
If you have equal volumes of water and air, the mass of the water is much greater than that of the air. In the
case of your water-jet engines, filling the pressurized air+water tank with water allows for a stronger reaction
force than if it were just filled with air, according to conservation of momentum (Newton's Second Law of
Motion). However, if too much water is added, it will reduce the space for the air and diminish the potential for
the generation of kinetic energy. So, what is the best amount of water? This is a question worth investigating
to find out.
Without water to be pumped
With water halfway full
Experiment | AIR+WATER power
(1) Build model 13 and operate it without any
water added to the water storage tank.
If no water is used, no water will enter the
pressurized air+water tank. The limited air in the
tank will rush out as soon as the nozzle is opened,
which leads to only a minor change in momentum
for the vehicle. As a result, the vehicle moves
forward at a slow speed for only a short distance.
(2) Fill the water tank half full of water and
operate the model again.
Add water into the water storage tank so it is
about half full. Use the pump to draw water into
the pressurized air+water tank, which pumps air
into the tank as well. At this time, since the volume
of the air+water tank remains the same, the air
density in the tank increases as more and more air
molecules are pumped into it. These air molecules
keep moving and colliding with each other even
when the pumping has stopped, and they generate
a constant total momentum, which always equals
the sum of the momentum of each molecule,
according to conservation of momentum. Using the
momentum formula: Momentum (Force) = Mass (M)
× Velocity (V), and Pascal's Principle: P (Pressure) =
F (Force) / A (Area), you can determine that when
the area is fixed, force will increase if the pressure
increases. Therefore, using the same air+water
tank (to keep the volume fixed), a greater force can
be derived to run the water-jet vehicle farther by
adding greater pressure.
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