Physics Reviews (A level)

Graaaaavity, don't mean too muuuuuch to me except in Physics A-level. Obviously gravity has already been very relevant in this subject, especially in the forces topics, but this topic delves even deeper into gravity.

Gravity relatively isn't a strong force, it only seems strong to us because we live on a fatass planet, which as you will see, makes a difference to the gravitational force. But force being weak is a bit of a callback to Quantum and the fundamental forces, so it is much weaker when compared strong nuclear, weak (despite the name) or EM.

EM is relevant though, as electric and magnetic are the other 2 types in the overall fields topic. Some things that are useful the fields in general are that force fields are non-contact. Fields can also be represented as vectors, as they are forces, so act in a direction.

There are also 2 kinds of fields, radial and uniform, and the names are quite descriptive.

Radial fields surround a round objects, so field lines point in or out of the object from all directions into the centre. An example of this would be an object with mass, so the gravitational field around it is radial.

Uniform fields are straight, with field lines acting parallel with each other, all pointing in the same direction. In gravitational, an example would be the surface of the earth relative to a person. This may seem odd, as the Earth's gravitational field is radial. However, when looking at the field from a relatively small scale, we can model the field as uniform, so e.g. gravity along a 5 metre stretch all acts down parallel with each other.

Gravity as a concept can be described as a universal attractive force between all matter. This means that all objects with mass have gravitational forces acting between them. However, from real life we know that 2 human-scale objects don't just move towards each other, which is because that force is really small. With larger massed objects, like planets, the force is more clearly experienced, like in orbits.

The equation to find the gravitational force between two point masses is F=(GMm)/r². G is the Gravitational Constant (6.67 × 10⁻¹¹), M and m are respectively the masses of the two objects we are finding the grav force between, and r is the distance between the masses. This equation is AKA Newton's Universal Law of Gravitation, if you were wondering. Another slightly notable part is that the relationship between the distance and force is the inverse square law. Also, since it is made to find the force between 2 point masses, this uses the centre of mass as where you start the distance from.

Now we move onto the gravitational field strength (little g). This is what we use to find the weight of something, as we times this by the mass. We know that on Earth, g is 9.81 (or 10 if you're a wrongun). We also often use g in F=ma equations. We use a very similar equation in this topic, which is g=F/m. This is the same thing, as acceleration due to gravity=gravitational field strength. So with this equation you can find g of object A, with the gravitational force of object A that acts on the mass of object B. An example would be finding g of Earth, using the Earth's gravitational force on a person with known mass. Also note that the gravitational force will change depending on the distance away, which is where the next part comes in handy.

Using this equation, you would find another equation for g by substituting in the gravitational force equation for F. If you work it out, then small m cancels out, leaving you with g=GM/r². From this equation, you can see that g of an object depends on M, the mass of the object we are finding the g. Make sure not to mix up M and m, or you will find the gravitational field strength of the smaller object, like the g of a person who is on earth rather the g of earth.

But you can also see that g depends on r, the distance away from the centre of mass. We always use 9.81 as g on Earth, however more precisely, that is g at the surface of earth. This means that if r decreases (getting closer to the centre), g gets stronger, and as r increases (further away), g decreases. This makes sense though, as as you get further from Earth, the weaker gravity strength will be. This equation is for use in radial fields. You can also see this effect when visualising radial fields, as the field lines spike out radially, so closer to the centre the distance between lines is smaller, which means that g is stronger. As you get further away, the lines are further away from each other, and g is smaller.