You can find all the walkthroughs in the following playlist :

https://www.youtube.com/playlist?list=PLoi8XQWRo3pIaA6i7ydKDxDE2UzS4QQBo


This is the walkthrough 1 :

Geocentric Universe
Unlocking the Aristotle Door
You can find this achievement in the Walkthrough 3 :
https://youtu.be/glE03S44ttA?list=PLoi8XQWRo3pIaA6i7ydKDxDE2UzS4QQBo


Spherical Earth
Unlocking the Zipline
You can find this achievement in the Walkthrough 6 :
https://youtu.be/8x9xYxyf6Pw?list=PLoi8XQWRo3pIaA6i7ydKDxDE2UzS4QQBo


Copernican Universe
Unlocking the Cavern Door
You can find this achievement in the Walkthrough 9 :
https://youtu.be/gbuIe391PQI?list=PLoi8XQWRo3pIaA6i7ydKDxDE2UzS4QQBo


Galilean Universe
Rotating the Bridge from Island 2 to Island 1
You can find this achievement in the Walkthrough 10 :
https://youtu.be/F800kNVeskM?list=PLoi8XQWRo3pIaA6i7ydKDxDE2UzS4QQBo


Simple Pendulum Physics
Unlocking the Safe
You can find this achievement in the Walkthrough 14 :
https://youtu.be/R4Cb8jihmGY?list=PLoi8XQWRo3pIaA6i7ydKDxDE2UzS4QQBo


Caves
Opening the Caves Scene
You can find this achievement in the Walkthrough 15 :
https://youtu.be/kF2llIUf4zs?list=PLoi8XQWRo3pIaA6i7ydKDxDE2UzS4QQBo


Train
Unlocking the Wagon
You can find this achievement in the Walkthrough 15 :
https://youtu.be/kF2llIUf4zs?list=PLoi8XQWRo3pIaA6i7ydKDxDE2UzS4QQBo


Have fun !