We’ve seen that all masses experience a gravitational force towards all other masses.

The magnitude of the gravitational force is given by this equation. 

And the direction of that force is always towards the other mass.

Now, another way of understanding gravity is to say that all masses have a gravitational field around them…

Which, as we saw in the Introduction to Fields, is… 

A gravitational field is a region where any mass experiences a non-contact force.

For a spherical mass like the Earth, we’ve already seen that the gravitational field lines look like this. 

And the closer together the lines are at a particular point, the stronger the field is at that point.

But what do we mean by “field strength” or “the strength of the field”?

Well, gravitational field strength is defined as gravitational force per unit mass, at a specific point in a gravitational field. 

And it’s a vector… 

The direction of the gravitational field strength vector is the same as the direction of the gravitational force vector. They both point towards the source mass.

So, if we take a mass like a satellite and move it to a point with higher gravitational field strength…we know that the gravitational force that the satellite experiences will increase. 

And what if we replace the satellite with a more massive object, like an asteroid?

Well, we know that the asteroid will experience…

If we replace the satellite with a more massive object, like an asteroid, we know that the asteroid will experience a larger gravitational force than the satellite.

However, field strength is force per unit mass, so the field strength doesn’t change! 

The gravitational field strength around a mass like a planet depends on the planet’s mass… 

But not on the mass of the object that’s in the gravitational field. 

So, in summary…

All masses are surrounded by a region where other masses experience a non-contact force. 

This region is called a…

This region is called a gravitational field.

And the gravitational field strength is defined as the… 
The gravitational field strength is defined as the gravitational force per unit mass that a mass would experience in a gravitational field.