Orbital energy of object overcoming Moon's gravity

In summary: Newton's Third Law of Motion dictates that the force between two masses is proportional to the product of their masses and the distance between them.
  • #1
salmayoussef
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Homework Statement



If the mass of the Moon is 7.4 x 1022 kg and its radius is 1.74 x 106 m, compute the speed with which an object would have to be fired in order to sail away from it, completely overcoming the Moon’s gravity pull.

This might seem like a stupid question to ask, but I'm not sure what equation to use. That's my only problem.

My classmate used V = √(2GM/r) but I'm not sure how he got it. Any help would be great!
 
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  • #2
Let's find the minimum speed necessary.

With the minimum speed, the object will just barely overcome the gravity, and so it will have zero velocity and zero gravitational potential energy.

Zero gravitational potential energy happens at an infinite distance (when you're "out of reach" of the moon's gravity). So the velocity of the object will approach zero as the distance approaches infinity.

If the kinetic energy becomes zero and the GPE becomes zero (you could have nonzero KE, but we're solving for the minimum) then (according to Conservation of Energy) what must the KE be when you're on the surface of the moon (where GPE is not zero)?
P.S.
I'm sorry if this was an awful hint. (I can't think of the best way to explain it, perhaps someone else will do better)
 
  • #3
Nathanael said:
Let's find the minimum speed necessary.

With the minimum speed, the object will just barely overcome the gravity, and so it will have zero velocity and zero gravitational potential energy.

Zero gravitational potential energy happens at an infinite distance (when you're "out of reach" of the moon's gravity). So the velocity of the object will approach zero as the distance approaches infinity.

If the kinetic energy becomes zero and the GPE becomes zero (you could have nonzero KE, but we're solving for the minimum) then (according to Conservation of Energy) what must the KE be when you're on the surface of the moon (where GPE is not zero)?

I appreciate the attempt, but I'm pretty clueless! Tried writing out your hint but I'm still not getting it... :cry:
 
  • #4
salmayoussef said:
I appreciate the attempt, but I'm pretty clueless! Tried writing out your hint but I'm still not getting it... :cry:

Do you know the formula for GPE? (The integral of gravity with respect to distance)

Edit:
Nevermind, I'm probably going to end up confusing you (not because it's a confusing problem, I'm just not a good teacher)Basically the energy of the object (Kinetic plus GPE) must be zero on the surface of the moon (the reason, which I tried to explain in my last post, is basically just that "GPE=0" is what it takes to overcome gravity, and "KE=0" means after it's overcome gravity it has no KE left (and so that would be the minimum velocity needed to escape gravity). And the energy is the same the whole time (on the surface and once it's escaped) so you can figure out the velocity that way.)

I'm still making it complicated hahaha I just really can't think of how to say it :\
 
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  • #5
Isn't it Egp = mgh ?
 
  • #6
salmayoussef said:
Isn't it Egp = mgh ?

Yes (well it's actually negative mgh)

What would "h" be in this situation?

And what would "g" be?

P.S. I edited my last post
 
  • #7
Nathanael said:
What would "h" be in this situation?

I think I understand what you're trying to say. Wouldn't h be the radius?
 
  • #8
salmayoussef said:
I think I understand what you're trying to say. Wouldn't h be the radius?

Yes, and what would "g" be?
 
  • #9
Nathanael said:
what would "g" be?

Fg = Gm1m2/r2, no?
 
  • #10
salmayoussef said:
Fg = Gm1m2/r2, no?

Yes, but g is not the force, it's just the acceleration. So it would only involve the mass of the moon.

Then you multiply it by h (r) and m and you get GPE=-Gm1m2/r

Then you set GPE+KE = to zero and therefore KE = |GPE| and then solve for V and you get the formula your classmate told you (sorry for being so direct but the explanation (of why GPE+KE=0) is difficult for me to articulate, I can't do much better than my last posts)

Edit:
There is logic behind it, though!
 
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  • #11
Nathanael said:
The explanation (of why GPE+KE=0) is difficult for me to articulate, I can't do much better than my last posts)
No, no, no! That made lots of sense actually! Had to read it over a few times but I got it! Thanks so much for your help. I appreciate it. :)
 
  • #12
salmayoussef said:
No, no, no! That made lots of sense actually! Had to read it over a few times but I got it! Thanks so much for your help. I appreciate it. :)

No problem, I'm glad you got something out of it!
 
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  • #13
salmayoussef said:
Isn't it Egp = mgh ?

The formula GPE=mgh is applied only when the gravitational force is constant. It is true enough near the surface of the Earth where the force of gravity is mg, with g about 9.8 m/s2.

At longer distances you need Newton's Law for Universal Gravitation.

Every point mass attracts every single other point mass by a force pointing along the line intersecting both points. The force is proportional to the product of the two masses and inversely proportional to the square of the distance between them:

[tex] F = G \frac{m_1 m_2}{r^2}\ [/tex],

where:

F is the force between the masses,
G is the gravitational constant,
m1 is the first mass,
m2 is the second mass, and
r is the distance between the centers of the masses.

The gravitational potential energy at distance r between two objects is [tex] GPE = - G \frac{m_1 m_2}{r}\ [/tex]

It is negative, and becomes zero when the objects are infinite far from each other.
As Nathanael explained, the total energy of the object must be at least zero to be able to escape from the Moon's gravity: E=KE+PE=0. On the surface of the Moon that means

[tex]0.5 m v^2-G\frac{mM}{R}=0[/tex]
where M is the mass of the Moon, and R is its radius.

ehild
 
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FAQ: Orbital energy of object overcoming Moon's gravity

What is the orbital energy of an object overcoming the Moon's gravity?

The orbital energy of an object overcoming the Moon's gravity is the amount of energy required for the object to maintain a stable orbit around the Moon. This includes both the kinetic energy of the object's motion and the potential energy of its position relative to the Moon's gravitational field.

How is the orbital energy of an object calculated?

The orbital energy of an object is calculated using the formula E = -GmM/2r, where G is the universal gravitational constant, m and M are the masses of the object and the Moon respectively, and r is the distance between the two objects.

What factors affect the orbital energy of an object overcoming the Moon's gravity?

The orbital energy of an object overcoming the Moon's gravity is affected by the mass of the object, the mass of the Moon, and the distance between the two objects. Other factors such as the shape of the object's orbit and external forces (such as atmospheric drag) may also play a role.

How does the orbital energy of an object change as it gets closer to the Moon?

As an object gets closer to the Moon, its orbital energy decreases. This is because the gravitational force between the object and the Moon increases, causing the object to accelerate and lose kinetic energy. Additionally, the object's potential energy also decreases as it moves closer to the Moon's surface.

Can the orbital energy of an object overcoming the Moon's gravity be increased?

Yes, the orbital energy of an object can be increased by either increasing its speed or increasing its distance from the Moon. However, it is important to note that any changes to an object's orbital energy must be carefully calculated to ensure that the object does not escape the Moon's gravity or collide with its surface.

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