Table and masses
Two masses are attached to the ends of a string going through a hole in a
table. Mass m moves on the table and while mass M hangs
vertically. Friction is negligible. It is further assumed that M
moves along the vertical and that neither mass goes through the hole
(because the string is long enough and the initial velocity of mass m
is not purely radial). The simulation allows displaying the true motion of
the masses, as well of that of the equivalent one-dimensional problem.
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The unit mass is the mass m of the particle on the table.
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The unit length is uL = (L2/m2g)1/3,
where L is the angular momentum of particle m around the
hole.
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The unit time is uT = (L/mg2)1/3.
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In these units we have m = L = g = 1 and the
total mass is 1+M/m.
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Gravitational energy is 0 when m is in the hole.
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On the right the effective potential energy
Mr/m+1/2r2
is displayed in red. You may use the mouse (or the
controls below) to select the mechanical energy E and the
initial polar distance r.
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The point of view of the three-dimensional projection can be changed
with the mouse.
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Each whole image can be moved with the mouse while pressing Ctrl.
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To change the zoom in the projections, press Shift
when moving up or down the mouse pointer.
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Put the mouse pointer over an element to get the corresponding tooltip.
Activities
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Use the polar coordinates r and φ of the mass m
to write the Lagrangian.
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Check that φ is cyclic and that the corresponding constant
of motion is L. Use the latter to write the one-dimensional
problem describing the radial evolution. Check the expression of the
effective potential energy.
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How many circular orbits (for m) there exist for each value of L?
Are they stable? Use the simulation to check your answers.
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Is there any unbounded orbit?
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Find a periodic non-circular orbit around E = 1.9
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If the minimum and maximum distances between the hole and the mass m
are, respectively, uL/2 and uL,
which is M/m? Use the program (after enabling Limits) to
check your computation.
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What happens if they are 3uL/8 and 3uL/4?
This is an English translation of the Basque original for a course
on mechanics, oscillations and waves.
It requires Java
1.5 or newer and was created by Juan
M. Aguirregabiria with Easy
Java Simulations (Ejs) by Francisco
Esquembre. I thank Wolfgang Christian and Francisco Esquembre for
their help.