Universal Gravitation problem, help with final statement.

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Homework Help Overview

The discussion revolves around a problem in universal gravitation involving the gravitational forces acting on a rocket situated between the Earth and the Moon. The original poster provides specific masses for the Earth and Moon, the distance between them, and the position of the rocket, seeking to determine the net gravitational force on the rocket.

Discussion Character

  • Conceptual clarification, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the calculation of gravitational forces exerted by the Earth and Moon on the rocket, with the original poster attempting to clarify their final statement regarding the net force. Questions arise about the definition of "net force" and the correct way to express directionality in terms of gravitational pull.

Discussion Status

Participants are actively engaging in clarifying the concept of net force and its representation. There is a recognition of the need to specify a single net force direction, with some participants suggesting that the Earth’s gravitational pull is dominant. The original poster expresses confusion regarding the wording of their final statement but acknowledges their calculations.

Contextual Notes

There are indications of fatigue affecting the original poster's clarity in communication, and the discussion includes a focus on ensuring that the final statement accurately reflects the net gravitational force without referencing a diagram.

Physics345
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Homework Statement


earth has a mass of 5.98x10^24 kg and the moon has a mass of 7.35x10^22 kg. The distance from the centre of the moon to the centre of the Earth is 3.84x10^8 m. A rocket with a total mass of 1200 kg is 3.0x10^8 m from the centre of the Earth and directly in between Earth and the moon. Find the net gravitational force on the rocket from the Earth and moon.

Homework Equations


Fnet=Gm1m2/r^2 + Fnet=Gm2m3/r^2

The Attempt at a Solution


Rocket to earth:
Fnet=5.32N=5.3N
Rocket to the moon:
Fnet=5.32N-0.833
Fnet=4.48N=4.5N rounded
Therefore, the net force of the rocket to the moon is 4.5N Right and the net force from the rocket to Earth is 5.3N Left.

I was just wondering if my Final statement is correct.
 
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Physics345 said:
Fnet=Gm1m2/r^2 + Fnet=Gm2m3/r^2
Physics345 said:
the net force of the rocket to the moon
You do not seem to understand "net force". It is the sum of all the applied forces, so there is only one net force on the object.
Physics345 said:
4.5N Right
Left/right depends on your diagram. Better to write in terms of "towards moon" or "towards Earth".
 
Fnet means the total/sum of all forces. That was a typo I meant to put FgE= Gm1m2/f13 FgM=Gm2m3/f23 Fnet=FgE-FgM fatigue is very intoxicating but thanks I figured out the answer to my question anyways. The answer is 4.5N Right towards the Moon and 4.5 Left towards Earth.
 
Last edited:
Physics345 said:
Fnet means the total/sum of all forces. That was a typo I meant to put FgE= Gm1m2/f13 FgM=Gm2m3/f23
Ok, but my comment still applies to your answer. You are asked for the net force; that's just one force, not a separate force towards each attractor.
 
Sorry I edited my comment, I'm just really tired I edited as you replied, so I can see how you came to that conclusion.
 
Physics345 said:
4.5N Right towards the Moon and 4.5 Left towards Earth.
Better, but as I wrote, left and right refer to some diagram (that I cannot see). Just write that it is 4.5N towards ... which one?
 
Oh Ill upload it now
 
Physics345 said:
Oh Ill upload it now
No, I don't need the diagram. The point is that you should not be referencing a diagram in your answer.
 
haruspex said:
No, I don't need the diagram. The point is that you should not be referencing a diagram in your answer.
Oh, I should be stating 4.5N Towards the Earth and 4.5N Towards the moon. Wow I feel silly, left and right shouldn't even be stated in this scenario.
 
  • #10
Statement revised, here is my new final statement. Therefore, the net force of the rocket towards the Moon is 4.5N, and 4.5N towards the Earth.
 
  • #11
Physics345 said:
Statement revised, here is my new final statement. Therefore, the net force of the rocket towards the Moon is 4.5N, and 4.5N towards the Earth.
There is a single net force on the rocket. You can specify it as a magnitude and direction. Magnitudes are non-negative by definition. Direction is either towards the moon or towards the Earth. Which?
 
  • #12
haruspex said:
There is a single net force on the rocket. You can specify it as a magnitude and direction. Magnitudes are non-negative by definition. Direction is either towards the moon or towards the Earth. Which?
Towards the Earth because the Earths gravitational pull is pulling the rocket!
 
  • #13
Physics345 said:
Towards the Earth because the Earths gravitational pull is pulling the rocket!
They are both pulling the rocket. Which wins?
 
  • #14
haruspex said:
They are both pulling the rocket. Which wins?
I would have to say the Earth wins, considering that the gravitational pull of the Earth is about 83% greater than the moons even after taking the altitude of the rocket into account.
 
Last edited:
  • #15
Wow it says it in my answer, the pull of the Earth on the rocket is 5.3N as opposed to the moon, which has a measly 0.8338N.
 
  • #16
Physics345 said:
Wow it says it in my answer, the pull of the Earth on the rocket is 5.3N as opposed to the moon, which has a measly 0.8338N.
Right!
So why did you write this:
Physics345 said:
4.5N Right towards the Moon and 4.5 Left towards Earth
and this:
Physics345 said:
the net force of the rocket towards the Moon is 4.5N, and 4.5N towards the Earth.
(which would add to zero net force) and this:
Physics345 said:
because the Earths gravitational pull is pulling the rocket!
?
 
  • #17
The answer was in my face this whole time I feel very dumb. Considering the lack of sleep I've had lately it's reasonable, well I probably should head to bed and catch up on some needed sleep. I'm sure this was frustrating for you, I can just imagine... I was able to do the math perfectly but I couldn't get the wording out for the final statement. Thanks a lot though I really appreciate it. Note,I wrote all that because I'm honestly being brain dead right now. I can write out all the math flawlessly my issue is wording when it comes to science I over think things and over complicate science a lot. But with more studying and practice I should have all this down to the point you'll see me on here helping people out =). If you want I could type out all the math I wrote but I decided to type out as little as possible because I was tired, but now that I want to prove a point here's the math:
FgE= Gm1m2/F13 FgM=Gm2m3/F23
Fnet=FgE - FgM
F13(distance Earth to rocket)=3.0 x 10^8
F23(distance rocket to earth)=(3.84 x 10^8) - (3.0 x 10^8)= 8.4 x 10^7
Fnet=(6.67 x 10^-11)(5.98 x 10^24)(1.2 x 10^3)/(3.0 x 10^8)^2 - (6.67 x 10^-11)(1.2 x 10^3)(7.35 x 10^22)/(8.4 x 10^7)^2
Fnet=5.32N - 0.8338N
Fnet=4.5N
 
Last edited:
  • #18
Physics345 said:
The answer was in my face this whole time I feel very dumb. Considering the lack of sleep I've had lately it's reasonable, well I probably should head to bed and catch up on some needed sleep. I'm sure this was frustrating for you, I can just imagine... I was able to do the math perfectly but I couldn't get the wording out for the final statement. Thanks a lot thought I really appreciate it. Note,I wrote all that because I'm honestly being brain dead right now. I can write out all the math flawlessly my issue is wording when it comes to science I over think things and over complicate science a lot. But with more studying and practice I should have all this down to the point you'll see me on here helping people out =). If you want I could type out all the math I wrote but I decided to type out as little as possible because I was tired, but now that I want to prove a point here's the math:
FgE= Gm1m2/F13 FgM=Gm2m3/F23
Fnet=FgE - FgM
F13(distance Earth to rocket)=3.0 x 10^8
F23(distance rocket to earth)=(3.84 x 10^8) - (3.0 x 10^8)= 8.4 x 10^7
Fnet=(6.67 x 10^-11)(5.98 x 10^24)(1.2 x 10^3)/(3.0 x 10^8)^2 - (6.67 x 10^-11)(1.2 x 10^3)(7.35 x 10^22)/(8.4 x 10^7)^2
Fnet=5.32N - 0.8338N
Fnet=4.5N
OK!
Get some rest...
 
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Likes   Reactions: Physics345
  • #19
HaHa, thanks again. Good night!
 

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