Motion of parallel-incoming particles in gravitational field

AI Thread Summary
The discussion centers on determining the fraction of a parallel beam of incoming particles that will impact a planet in a gravitational field. Key points include the application of energy conservation, leading to a velocity equation at impact, and the use of angular momentum conservation for tangential velocity calculations. However, challenges arise due to the difficulty in evaluating the distance vector as it approaches infinity. Participants agree that additional information, such as the diameter of the particle beam, is necessary to accurately calculate the fraction of particles that will reach the planet. Overall, the problem requires clarification on the beam's dimensions to proceed with a solution.
Aaronaut
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Question:
There is a large parallel beam of incoming particles with mass m and uniform velocity v0 (v0≪c) in the presence of a gravitational field of a (spherical) planet with mass M and radius R. (without GR) The question is what fraction of the particles will eventually arrive at the planet.
Relevant formulas/attempt to solve
The conservation of Energy implies that
v20=v2−γMR,
which would give the norm of the velocity at the moment of impact. Furthermore, for the tangential component of the velocity could be at least in principle calculated using the conservation of angular momentum,
rv0,t=Rvt,
where r is the original distance vector of the particle. However, there is the problem, that since the case of a far-away particle is considered, r→∞, the left-hand expression is hard to evaluate. In addition, these equations still do not, at least from my point of view, contain enough information to calculate the fraction of particles arriving.
 
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Not sure I'm interpreting the question correctly. Are we to assume that the diameter of beam exceeds that of the planet, so it's a question of which particles would have a perigee closer than the radius of the planet? If so, I agree there's not enough information. We need to know the diameter of the beam.
 
I think the problem is to determine the radius of the particle beam, because it should be proportional to the number of particles.
 
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