A simple but
Using the same classroom elements employed to exhibit the conservation of linear momentum, we can demonstrate a simple method that in a VERY NON-INTUITIVE manner allows a closed system to apparently disregard said law of conservation of linear momentum.
The results of the experiment open a thought-provoking discussion with interesting practical applications.
What we need.
A low friction track and cart (fig 1) common in many physics classrooms (air track type not suitable for this particular experiment).
The low friction track and its cart must itself be on a low friction surface (fig 2) so that both the track and the cart can move horizontally independently with minimum friction between them.
The desired effect can also be observed simply by putting the low friction track on a skateboard (or other low friction surface figs 3 and 4).
The experiment can also be performed on low friction rails, water (messy) or dry ice plucks.
Description of the setup.
We have the low friction track (1) on a low friction surface (2) with a low friction chart (3) on top.
There is a tensed spring (4) between the chart (3) and the track’s (1) +X side, the spring will be released by a trigger mechanism on command (Fig 5).
Fig 5 (system to be tested)
Test 1, release the spring with no air brake
When the spring is released (fig 6) it pushes with force F1 against the low friction track or mass 1 (M1) giving it an acceleration in the +X direction so that it gains a +X velocity of V1, an equal force F2 is exerted against the cart or mass 2 (M2) so that it gains a velocity (V2) in the –X direction.
When the cart (M2) collides with the track’s –X end (5) it exerts a force F4 that stops the track’s horizontal velocity, an equal and opposed force F3 stops the cart and all elements return to their original state (0 velocity).
F4 (and therefore F3) are equal to forces F1 and F2 because cart M2 travels from +X to –X at a CONSTANT velocity, as cart M1 travels from –X to +X also at a constant velocity.
Test 2, release the spring with an air brake attached to mass M2 (the cart).
We now place an air brake (6) (small parachute) on cart M2 and repeat the experiment.
Again when the spring is released it pushes with force F1 against the low friction track/ mass 1 (M1) giving it an acceleration in the +X direction so that it gains a +X velocity of V1 that is CONSTANT as mass M1 travels in the +X direction.
The cart/mass 2 (M2) gains an INITIAL velocity (V2i) in the –X direction that DECREASES (because of the air brake’s drag) without affecting the +X velocity of mass M1.
So while M1’s +X velocity is constant, M2’s –X velocity decreases.
Because the cart’s (M2) final velocity (V2f) is LESS than its initial velocity (V2i), force F4 (F4=V2f/M2) is not equal to the force that gave M2 its initial velocity (V2i), therefore M1’s momentum is greater than M2’s momentum and the track/M1 will retain a +X velocity (RV1), it has accelerated.
Up to now there is nothing unexpected in the experiment’s results.
Test 3, enclose masses M1 and M2 in an aerodynamic airtight cover.
Now comes the interesting part, the apparently controversial part, we enclose the low friction track/ mass 1 (M1) with an airtight covering (1) and repeat the experiment with and without the air brake
Without air brake.
When we execute the experiment without an air brake attached to mass M2 (figs 5, 6 and 7) mass M1 will go thru 3 distinct stages:
(1) At rest (fig 5)
(2) Spring is released and mass M1 gains velocity in the +X direction (fig 6)
(3) M1 stops when chart M2 collides against the inner –X side of M1 (fig 7).
With the air brake
When we execute the experiment with an air brake attached to mass M2 (figs 8, 9 and 10) mass M1 will behave a little differently:
(1) At rest (fig 8)
(2) Spring is released and mass M1 gains velocity in the +X direction (fig 9)
(3) As M2 (the chart) travels in the –X direction, drag on the air brake (little parachute) slows it’s –X velocity, the random collisions of air molecules do NOT interact directly on the inner surface of the airtight covering ( as the result of the experiment demonstrates)
(4) M2 collides against the inner –X side of M1 (fig 10) but as its velocity (therefore its momentum) has decreased it is not sufficient to stop the system and M1 continues moving in the +X
direction until friction stops it’s +X velocity.
But in a 0g and airless environment M1 will retain its velocity change in the +X direction and may increment that acceleration in every cycle:
For a bettor description of the cycle and practical application please see:
A simple description of the proposal using two astronauts and batman (Adam West version).
A step by step explanation of the principals involved (For the serious minded, no kids, no skateboard, no batman, no puns).
Article on Fortean Times magazine by David Hambling.
The aerodynamic airtight cover (box) must be of sufficient width so that the air brake has plenty of space for the air in the box to behave in a turbulent manner, if not the air brake will act as a piston compressing the air against the –X side of the box and the experiment will not give the described results.
We have found that the larger the box the better (fig a), a 3m (meter) x 0.5m x 0.5m with a 0.4m x 0.4m air brake (fig b) gets the describe results without problems when tested on low friction rails, water (messy) or dry ice plucks. (although the experiment described here is adequate for demonstration purposes , a more complex torsion balance apparatus using metrology similar to Henry Cavendish’s 1706 experiment to measure the force of gravity between masses is recommended for evaluating true thrust output).
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