What makes us feel heavy on Earth? What makes us feel heavy on Earth?
$F=m\cdot g$ is the gravitational force that earth exerts on us and $N$ is the force that the surface has on us (Normal force), so which one makes us feel heavy?
My professor claims that $N$ gives us this feeling, but I don't understand why. Could someone clarify or correct this for me?
 A: The ISS is in orbit, therefore very much under the force of terrestrial gravity, but its crew definitely feel weightless, because they are in free fall. Every bit of their body is moving at exactly the same speed, so there are not the same internal forces within their body as there are here on Earth.
On Earth, however, you do feel weight, because you are being pulled by gravity, but are being stopped from free-falling by the ground below at your feet. Your body, however, is still very much being accelerated downwards, so the ground has to stop you by exerting a normal force equal in strength to the gravitational interaction pulling you down.
Thus, your feet have to hold some weight, and every single layer of your body has to sustain the weight of the layers above. If you were in free fall, you would not feel these normal forces on any part of your body, you'd effectively feel weightless.
These internal forces are called stresses in continuum mechanics, and the study of how the structure of objects changes under these stresses is central to the subject.
An excellent example of these internal stresses is the Russian cosmonaut Anatoly Ivanishin, who is currently on the ISS. In space, his face looks a lot different than in does here on Earth.


Even while holding a close to neutral facial expression, you can see in space his cheeks stay a lot higher than they do on Earth. This really surprised me last week during the Crew Demo-2 SpaceX mission, because his face looked quite funny in space, and then I proceeded to search for images of him on Earth and the difference is quite apparent.
A: 
When an object (having mass) comes in contact with a surface under the application of net external force(s), the surface exerts a reaction force to stop the object moving through it. This reaction force (equal in magnitude but opposite in direction) makes the object to experience the reaction force (normal force) exerted by the surface. An object experiences pressure, force when it is pushed or stopped to move by a surface.
Similarly, you experience the gravity force $F_g=Mg$ exerted by the Earth towards its center & thus you have a tendency to move towards the Earth's center but the ground surface stops you by exerting an equal but opposite force called normal (reaction) force $N$. This makes you feel your body weight on your feet below at the ground i.e. you feel heavy on the Earth due to upward normal force $N$. (as shown in figure above, a body at rest experiencing heavy on the surface due to normal force $N$). You can feel only gravity force $Mg$ but not your weight until you are stopped by some surface or floor on which you are standing.
