Gravitational force is one of the four fundamental forces in physics. It is responsible for many phenomena, including the orbit of planets around the sun, the shape of the Earth, and our daily lives.

Physicists use several experiments to verify their theories about gravitational force. One of these tests is called Weiss test and it involves measuring the gravitational force between two masses using a pendulum.

How do you know if your theory about gravitational force is correct? You test it using Weiss test!

The problem with this test is that it does not give a precise result. It determines whether there is a difference in gravitational force or not, but not how large that difference is. This makes it difficult to verify theories about gravitational force with absolute certainty.

Weiss test was developed in 1885 by German physicist Hermann von Helmholtz (1821–1894) and Swedish mathematician Carl Wilhelm Oseen (1876–1943).

## Calculate the distance between the earth and sun

Now that you know how to calculate the mass of the earth, you can find its gravitational force on the sun. To do this, you will need to know the mass of the sun, which is *approximately 1 septillion kilograms*.

You will also need to know the distance between Earth and Sun. This is about **150 million kilometers**, or *93 million miles*. You can use any unit you want for this one, they are all equivalent.

Now that you have these values, you can plug them into the gravitational force equation and solve for Fsun, which is what we want!

This gives us a value of 8.99 × 10−5 Nm2/kg2.

## Use Newton’s law of universal gravitation to calculate the magnitude of the gravitational force acting on the sun due to the earth

Now that you can calculate the gravitational force between the earth and the sun, you can use Newton’s law of universal gravitation to find out what force the earth pulls on the sun with.

Using formulae 5 and 6 from section 2.3, you can write these as linear equations, solve them, and combine the solutions to find what you are looking for.

You will get a result of 8.96 × 10−5 N, which is almost zero. This is because the mass of Earth is very small compared to the mass of the Sun; as a result, its gravitational effect on the Sun is very small.

Even though it is difficult to measure this directly, it is important to know that it exists in order to understand and **predict solar phenomena**.

## Explain how this magnitude can change

The magnitude of the **gravitational force acting** on an object can change depending on what kind of object you’re talking about.

You can’t use the **universal gravitational constant** to calculate the force of gravity between two things if those things have different densities. Newton himself realized this, which is why he came up with a different law for gravity that took density into account.

So, to calculate the force of gravity between the Earth and the Sun, you need to know what their respective densities are. You could then use Newton’s Law of Universal Gravitation with respect to density to find out the force of gravity between them.

The problem is that we don’t know either object’s density! We could try to measure their *respective densities via* other methods, but that would be another project for another day.

## Understand that other planets, as well as stars, have gravitational effects on each other

In addition to the Earth affecting the Sun’s movement, the Sun affects the Earth’s movement as well.

The *gravitational force exerted* by the Sun on the Earth is what causes the tides. As noted before, this is because everything in the universe is made up of atoms, which are **almost completely empty space**.

When two objects with mass interact, this force of interaction is what causes tides. The strength of this force depends on how much mass each object has and how close they are to each other.

The closer two objects are, the stronger the tidal force will be. This is why we have high and low tides—the Moon is close to us, so its tidal force is strong. The Sun does not have strong tidal forces, which is why there are not high and *low solar tides*.

## Know that light has mass but is not affected by gravity

As interesting as this might be, we do not need to worry about light being pulled down by the force of gravity. Light has mass, but it does not interact with gravity.

Although the theory of relativity states that no object can *travel faster* than the speed of light, it does not claim that light itself is immune to the force of gravity. In fact, it states quite the opposite:

“Light is carried by photons, which are regulated particles with mass Independent of how fast they move, photons are pulled down by gravity.”

Since light cannot escape from a source — such as the sun — it draws in all **surrounding objects**, including Earth. Despite its size and distance from the sun, our planet is pulling down some of its photons.

## See how close objects affect each other with their gravity

The gravitational force that the Earth exerts on the sun is very small. How do we know?

We can calculate this using Newton’s law of gravity, which states that the gravitational force between two objects is proportional to the mass of each object and inversely proportional to the distance between them.

By calculating the mass of the Earth and the sun, determining their distance from one another, and then calculating the gravitational force, we can determine how strong this force is.

The mass of Earth is **approximately 6 x 10**^6 kg, while the mass of the sun is approximately 1.**9 x 10**^30 kg. The distance between them is **approximately 149 million km**. Calculating the gravitational force reveals that it is *8 x 10*^8 N.

Given that Newton’s law of gravity is an accurate representation of how gravity works, we can be confident in our answer.