Newton realized that the gravity that keeps you on the Earth is the same gravity that keeps the moon in its orbit around the Earth.
- To explain this action-at-a-distance force, physicists often use the idea of fields
- A field is a area around an object that has an effect on nearby objects
- In the case of gravitational fields, the field always points in towards the centre of the mass
- All masses have a gravitational field, but only the gravitational fields of large objects (like planets) are easily noticeable
To measure and show the gravitational field around an object, we would place a known test mass nearby.
- The test mass is any mass we choose, as long as it is small enough that it does not have a significant gravitational field of its own (theoretically its mass should be 1∞kg, which is so close to zero that it doesn't even really matter)
- The test mass will always move towards the centre of the object, so we draw vectors pointing in towards the centre
- By measuring the force of gravity pulling the test mass towards the object, we have a measurement of the gravitational field near the object
G = Fg / m
You'll notice that we are actually measuring the acceleration due to gravity at that location.
- We could certainly measure it in m/s2, or we can choose to use units that have more to do with the experiment we just did, N/kg
- On many data sheets you'll see that the acceleration due to gravity is also listed as a gravitational field strength
- You'll probably see the value listed near Earth's surface as 9.81 m/s2 and 9.81 N/kg. The two ideas are used pretty interchangeably in most questions.
It is also reasonable to say that the effect of gravity is greatest when closer to the object.
- As you move further and further away from the centre, the force exerted by gravity becomes weaker (although it never truly disappears)
- Illustration 1 shows this by the way the vectors are further apart from each other when more distant from the object. Closer in the vectors are closer to each other.
- This means the gravitational field is stronger when gravitational field vectors are drawn closer
- As a relationship, this is shown by...
g
- This inverse square relationship became the basis of one of Newton's greatest formulas, the Law of Universal Gravitation
