Maybe we should then show how these can all be combined to do simple operations - one example I have in mind is something like:

```
>>> from astropy.constants import G, M_sun
>>> F = G * (3. * M_sun) * (2. * u.kg) / (1.5 * u.au)**2
>>> F.to(u.N)
<Quantity 0.01581795428812989 N>
```