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Two body escape velocity 
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#1
Jul414, 07:42 AM

P: 67

After calculating the gravitational PE using :
PE = ( G * m1 * m2 ) / d Then i split the result into KE between the two bodies according to the ratio of the masses, then calculated the individual velocities from those (based on KE = ½ * m * v ²) Any comments ? 


#2
Jul414, 09:34 AM

P: 1,988

How did you split the KE between the two bodies?



#3
Jul414, 09:58 AM

P: 759

According to my calculation, using your method, v1=v2 regardless of the mass ratio. But I may be doing something wrong.
Keep in mind that according to the conservation of momentum and the third law of motion: m1v1 = m2v2 must be true. So if KE=25, then KE1=8.333 and KE2=16.666. I suck at math, so I have little confidence in my analogy. :) 


#4
Jul414, 10:08 AM

P: 1,988

Two body escape velocity
Why don't you use the conservation of momentum result? If the mass ratio is 1:2, the KE ratio will be 1:4. 


#6
Jul414, 11:33 AM

P: 1,988




#7
Jul414, 11:47 AM

P: 759




#8
Jul514, 06:25 AM

P: 67

The split of the KE between the two bodies, is based on the mass ratio :
So : KE (m1) = ( m2 / ( m1 + m2 ) ) * PE KE (m2) = ( m1 / ( m1 + m2 ) ) * PE Then the individual velocities from : v ( m1 ) = sqrt ( ( KE ( m1 ) ) / ( ½ * m1 ) ) v ( m2 ) = sqrt ( ( KE ( m2 ) ) / ( ½ * m1 ) ) Ive ran this through as an example and the equal momentum is preserved. Comments ? 


#9
Jul514, 06:53 AM

P: 67

Basically, in asking if the total KE of both objects is equal to the original PE, this inquiry is based on the widely used statement that the mass of m2 is irrelavent in the caculation of escape velocity.



#10
Jul514, 07:00 AM

HW Helper
Thanks
P: 10,769

Well, the gravitational potential energy is negative if it is zero at infinite separation, how do you split it into kinetic energy of two bodies?
ehild 


#12
Jul514, 10:03 AM

Mentor
P: 12,081

You can also see that m_{1}>>m_{2} means all the energy is used for m_{2} and v_{1} is negligible, in agreement with the usual definition of an escape velocity. 


#13
Jul714, 04:51 AM

P: 67

Thanks for your input everyone, heres some more : I read on wikipedia that " the barycentric escape velocity is independent of the mass of the escaping object "
However, according to my previous thinking, the mass of the escaping object does alter the barycentric escape velocity, though at small values its virtually unnoticable, given a large enough value it does count. Any comments ? 


#14
Jul814, 01:22 PM

Mentor
P: 12,081




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