# Finding gravity on a star/planet

• gigglin_horse
In summary, the mass of a certain neutron star is 3.0x10^30kg (1.5 solar masses) and its radius is 8000 m (8km). The acceleration of gravity at the surface of this condensed burned-out star is 3.12x10^12g.
gigglin_horse

## Homework Statement

"The mass of a certain neutron star is 3.0x10^30kg (1.5 solar masses) and its radius is 8000 m (8km). What is the acceleration of gravity at the surface of this condensed burned-out star?"

I have no idea where to start

Nothing!

So from that sight, I gather this:

F=Gx(m1m2/d^2) ...right?

So:
G=F/(m1m2/d^2)

meaning:
F/((3.0 x 10^30) x m2/8000^2) ... Whats m2 and F?

The m1m2 will get you the total attraction between both bodies. You only need one, so lose the m2 (just set it to 1).

You don't need to flip the equation around. You're solving for F. That's your answer.

G is the gravitational constant, which you are given on that page (6.67x10^-11).

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Ok, so...

F = (6.67x10^-11) x [ (3.0x10^30)/(8000^2) ]

so F = 3.1265625 x 10^12Isn't that a bit strong considering Earth is ^-11 and this star has only an 8km radius? Or am I looking too much into it?

But you should try to understand what you're doing with this equation, don't just plug the numbers in.

The force an object feels under gravity of a massive object is directly proportional to the mass of the massive object.
i.e.: F is proportional to m : as m gets bigger the force gets bigger

and inversely proportional to the square of its distance
i.e. F is proportional to 1/(d^2) : as d gets bigger the force get smaller - and quickly

Put those together and you've got F proportional to m / (d^2).

This is only proportional though, which means you have no units. The gravtitational constant allows you to convert it to units.
So:
F = G * m / (d^2)

I haven't checked the numbers but this is a neutron star, and you are on its surface. Millions or billions of g's is par for the course.

What you need to do is check every one of your numbers to make sure you've got the units right. Is everything in kg or metres, not grams or kilometres? That's a really easy mistake to make.

I'm sure its all in the correct units.
Thank you both for your help
=]

Well, isn't Earth (6.67x10^-11)?
So this star being (3.12x10^12) seems a bit extreme

gigglin_horse said:
Well, isn't Earth (6.67x10^-11)?
So this star being (3.12x10^12) seems a bit extreme
No. Earth is 9.8x10^0.

The 6.67x10^-11 is the universal gravitational constant.

Think of this:

Your car's engine revs are proportional to your car's speed (let's pretend) but to apply units to that you'd have to have a constant that says "X revs equals Y miles per hour". Say, maybe your constant is q=.05
i.e.: velocity = q * revs

revs = 1000

So, v = .05 * 1000
v = 50mph

the consant there is q

Last edited:

## 1. How do scientists find the gravitational force on a star or planet?

Scientists use the universal law of gravitation, which states that the gravitational force between two objects is directly proportional to their masses and inversely proportional to the square of the distance between them. By measuring the mass of the star or planet and its distance from another object, scientists can calculate the gravitational force.

## 2. Can gravity vary on different parts of a star or planet?

Yes, gravity can vary on different parts of a star or planet. This is because the gravitational force is affected by the distribution of mass within an object. For example, a planet with a denser core will have a stronger gravitational force at its center compared to its surface.

## 3. How is the gravitational force measured on a star or planet?

The gravitational force on a star or planet is measured using instruments such as a gravimeter or a seismometer. These instruments detect the effects of gravity on objects and can provide accurate measurements of the gravitational force.

## 4. Why is finding gravity on a star or planet important?

Understanding the gravitational force on a star or planet is important for a variety of reasons. It helps us understand the structure and composition of these objects, as well as their orbital dynamics. It also plays a crucial role in space exploration and can impact the development of technologies such as spacecraft and satellites.

## 5. Can gravity be created or manipulated on a star or planet?

Gravity cannot be created or manipulated on a star or planet. It is a fundamental force of nature that exists due to the presence of mass. While the effects of gravity can be counteracted through technologies such as artificial gravity in space, the force itself cannot be artificially created or altered.

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