Find the speed of a satellite at a distance R from Earth

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The discussion revolves around calculating the speed of a satellite at a distance R from Earth, focusing on gravitational forces and orbital mechanics. Participants clarify that the orbital velocity for a circular orbit is derived from equating gravitational force to centripetal force, leading to the formula v = √(GM/R). They emphasize that while the potential energy is conventionally negative, it is crucial to understand that the total mechanical energy can also be negative if the potential energy's magnitude exceeds that of kinetic energy. The conversation highlights the importance of using correct variables and understanding the reference points for potential energy in gravitational contexts. Ultimately, the total mechanical energy of the satellite is negative, indicating that work is required to escape Earth's gravitational influence.
  • #31
So, the total mechanical energy
= Ek + Ep = 1/2mv^2 + (-GmM/r) = gRm/4 - GmM/2R = -mgR/2. Is it?
 
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  • #32
Helly123 said:
So, the total mechanical energy
= Ek + Ep = 1/2mv^2 + (-GmM/r) = gRm/4 - GmM/2R = -mgR/2. Is it?
No. Do not mix g and G. You know that GM/R2=g, so GM=gR2.
 
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  • #33
ehild said:
No. Do not mix g and G. You know that GM/R2=g, so GM=gR2.
Yes
 
  • #34
Helly123 said:
Yes
So what result did you get?
 
  • #35
Helly123 said:
So, r = 2R
r is distance from satellite to Earth's center
Btw, mechanical energy is Ek. The answer is -mGR/4
How can Ek be negative?

NO. The Mechanical Energy is the sum of the Gravitational Potential Energy plus the Kinetic Energy. The Kinetic Energy may be positive, but the Mechanical Energy value can still be negative, provided the Potential Energy is negative, and larger in size than the Kinetic Energy.
 
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  • #36
Ok. My answer is like Ep + Ek
While the formula i know for Ep is -GMm/r
r = 2R
Ek as usual is 1/2mv^2
V = ##\sqrt{ gmR/2}##

Ep + Ek
-GMm/2R + mgR/4 = -mgR/2 + mgR/4 = -mgR/4
Is that it?
 
  • #37
PeterO said:
NO. The Mechanical Energy is the sum of the Gravitational Potential Energy plus the Kinetic Energy. The Kinetic Energy may be positive, but the Mechanical Energy value can still be negative, provided the Potential Energy is negative, and larger in size than the Kinetic Energy.
PeterO said:
NO. The Mechanical Energy is the sum of the Gravitational Potential Energy plus the Kinetic Energy. The Kinetic Energy may be positive, but the Mechanical Energy value can still be negative, provided the Potential Energy is negative, and larger in size than the Kinetic Energy.
I see
 
  • #38
Helly123 said:
Ok. My answer is like Ep + Ek
While the formula i know for Ep is -GMm/r
r = 2R
Ek as usual is 1/2mv^2
V = ##\sqrt{ gmR/2}##

Ep + Ek
-GMm/2R + mgR/4 = -mgR/2 + mgR/4 = -mgR/4
Is that it?
Yes.
 
  • #39
ehild said:
Yes.
Ok
 
  • #40
The potential energy is negative and twice the magnitude of the kinetic energy (which is positive), thus the total energy is negative. So work is required to escape the gravitational well of the Earth no matter where you are.
 

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