# I Little g without Big G

D

#### Deleted member 658453

Anyone know of a way to solve for gravity of a planetary body without using the gravitational constant?

Related Classical Physics News on Phys.org

#### Ibix

Use orbits of its moons or other satellites. That gives you $GM$ which you can then use to get $GM/r^2$ for any $r$. No need to find $G$ or $M$ independently.

D

#### Deleted member 658453

What if no satelites to measure?

#### A.T.

Anyone know of a way to solve for gravity ...
Solve what? You haven't stated what is given.

D

#### Deleted member 658453

It’s not a specific value calculation. I’m asking about method. Do you know of a way to find a body’s gravity, in equation form perhaps, without using the gravitational constant to find the answer.

#### Ibix

Any measure of $g$ at a known $r$ gives you $GM$ and lets you deduce $g$ at any given $r$ outside the body. You need to work quite hard to get $M$ and $G$ separately.

#### russ_watters

Mentor
It’s not a specific value calculation. I’m asking about method. Do you know of a way to find a body’s gravity, in equation form perhaps, without using the gravitational constant to find the answer.
You were given a method and you rejected it because you said the information needed wasn't available. Are you going to give the same response for any method given? What information is available? We can't answer the question if you won't tell us the constraints.

D

#### Deleted member 658453

You were given a method and you rejected it because you said the information needed wasn't available. Are you going to give the same response for any method given? What information is available? We can't answer the question if you won't tell us the constraints.
Where you coming from? I haven’t rejected anything. It’s a general question, not a specific problem. No other info needed.

#### A.T.

... without using the gravitational constant to find the answer.
But what can be used?

D

#### Deleted member 658453

that’s what i’m asking...do you know any other parameter combinations, any other method of any nature, any other equation that finds gravity without big G?

#### russ_watters

Mentor
Where you coming from? I haven’t rejected anything. It’s a general question, not a specific problem. No other info needed.
You rejected the use of satellite trajectory data.

For any problem you want to solve, you have information you know and an answer you want to find. The information you know may or may not be sufficient to find the answer.

You've told us what you want to find, but nothing at all about what you know, so someone made a guess. And you responded, no, we don't know that.

I can think of several other methods requiring different known data. If I post them, will you tell me that, no, we don't have that information either? Why don't you save us the time and give us more of the constraints that you aren't telling us.

Or, if the basic constraint is that we know nothing then the answer is easy: we can't find g.

D

#### Deleted member 658453

You’re a staff mentor? Dude. You have a terrible disposition. I didn’t reject satellites. I merely asked what if there are none, to draw out more responses. Where did I respond “no we don’t know that”? If you can think of several other methods that require different known data then list them because that’s exactly what I want to know. If you can’t do that just move along.

#### A.T.

If you can think of several other methods that require different known data then list them because that’s exactly what I want to know.
Drop something and measure its acceleration.

Last edited:

Staff Emeritus
You’re a staff mentor? Dude. You have a terrible disposition.
And you're awfully demanding when it comes to free help.

Ibix is 100% right. It is far easier to measure GM than G, and GM gives you g vs. r. Russ is 100% right. If you don't have anything you can see move in the gravitational field you have no way to measure it.

D

#### Deleted member 658453

How in the world have I been demanding? Point to one phrase where I have demanded anything.

#### Ibix

The problem is that you are asking an incredibly open question. The simple answers are to look at orbits (dropping something is a subset of this) or weigh something of known mass. But we could go on spinning out wilder and wilder ways of measuring $g$ in ever more complex ways probably forever.

So although V50 and Russ are being blunter than I would be, they are asking the next question I would ask: why do you want to know? I very much doubt that an exhaustive list of possible experimental methods could ever be written. The obvious ones are above.

D

#### Deleted member 658453

Thanks for your reply, Ibix. I wasn’t looking for any wild manifestations of the task. Just simply if anyone new another way. Perhaps it’s too simple a question for the cerebrally afflicted, and before anyone jumps on me again, that’s a compliment. You’re the first to ask why, so I’ll answer. I’ve found a way to calculate any body’s gravity (the acceleration variety) independent of the gravitational constant and just wanted to make sure it is unique before submission for print.

#### PeterDonis

Mentor
I’ve found a way to calculate any body’s gravity (the acceleration variety) independent of the gravitational constant and just wanted to make sure it is unique before submission for print.
PF is not for discussion of personal speculations or original research.

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving