How do we define a Quantity and which parts does it have?

In [1]:

```
from astropy import units as u
```

In [2]:

```
# Define a quantity length
# print it
```

In [3]:

```
# Type of quantity
```

In [4]:

```
# Type of unit
```

In [5]:

```
# Quantity
```

In [6]:

```
# value
```

In [7]:

```
# unit
```

In [8]:

```
# information
```

Quantities can be converted to other units systems or factors by using `to()`

In [9]:

```
# Convert it to: km, lyr
```

We can do arithmetic operations when the quantities have the compatible units:

In [10]:

```
# arithmetic with distances
```

Quantities can also be combined, for example to measure speed

In [11]:

```
# calculate a speed
```

In [12]:

```
# decompose it
```

- Convert the speed in imperial units (miles/hour) using:

```from astropy.units import imperial``` - Calculate whether a pint is more than half litre

You can compare quantities as comparing variables.

Something strange? Check what deffinition of pint astropy is using. - Does units work with areas? calculate the area of a rectangle of 3 km of side and 5 meter of width. Show them in m^2 and convert them to yards^2

In [13]:

```
#1
```

In [14]:

```
#2
```

In [15]:

```
#3
```

Many units are compositions of others, for example, one could create new combinationes for ease of use:

In [16]:

```
# create a composite unit
```

In [17]:

```
# and in the imperial system
```

and others are already a composition:

In [18]:

```
# what can be converted from s-1?
```

In [19]:

```
# or Jules?
```

In [20]:

```
# Unity of R
```

Sometime we get *no units* quantitites

In [21]:

```
# no units
```

What happen if we add a number to this?

In [22]:

```
# arithmetic with no units
```

In [23]:

```
# final value of a no unit quantity
```

Some conversions are not done by a conversion factor as between miles and kilometers, for example converting between wavelength and frequency.

In [24]:

```
# converting spectral quantities
```

In [25]:

```
# but doing it right
```

Other built-in equivalencies are:

`parallax()`

- Doppler (
`dopplr_radio`

,`doppler_optical`

,`doppler_relativistic`

) - spectral flux density
- brigthness temperature
- temperature energy
- and you can build your own

In [26]:

```
# finding the equivalencies
```

In [27]:

```
# but also using other systems
```

In [28]:

```
# Printing values with different formats
```

Quantities can also be applied to arrays

In [29]:

```
# different ways of defining a quantity for a single value
```

In [30]:

```
# now with lists
# and arrays
# and its arithmetics
```

In [31]:

```
# angles are smart!
```

To work nicely with matplotlib we need to do as follows:

In [32]:

```
# allowing for plotting
from astropy.visualization import quantity_support
quantity_support()
# loading matplotlib
%matplotlib inline
from matplotlib import pyplot as plt
```

In [33]:

```
# Ploting the previous array
```

We want to have functions that contain the information of the untis, and with them we can be sure that we will be always have the *right* result.

In [34]:

```
# Create a function for the Kinetic energy
```

In [35]:

```
# run with and without units
```

- Create a function that calculates potential energy where *g* defaults to Earth value, but could be used for different planets. Test it for any of the *g* values for any other planet.

In [36]:

```
#4
```

In [37]:

```
# run it for some values
```

In [38]:

```
# on Mars:
```

Some times we want to create our own units:

In [39]:

```
# Create units for a laugh scale
```

- Convert the area calculated before `rectangle_area` in Hectare (1 hectare = 100 ares; 1 are = 100 m2).

In [40]:

```
#5
```