How exactly does the ground “push back” against gravity?
As you can see in Figure 3.25, the ground prevents you from falling because it pushes upward on you, so that the force of gravity pulling you downward is precisely balanced by the upward force from the ground. But how exactly does the ground push upward? The answer is that the atoms and molecules in solid objects resist being pushed together, which leads to balance as follows:
- The downward force of gravity is your weight. (Mathematically, this force is your mass times the acceleration of gravity, or mg.)
- The greater your weight, the harder you are pushing down on the ground, and therefore the more your weight is trying to push the atoms and molecules in the ground together.
- Those atoms and molecules therefore push back, which in this case means it is pushing upward on you. As long as the ground does not crack or open up, the upward push will automatically balance your weight pushing down.
In other words, as gravity pulls you downward against the ground (or any other solid object, like a chair or table or floor in a tall building) , the atoms and molecules of the ground automatically push back against you. That is the reason you can stand on solid ground.
This box explains the basic reason why the ground pushes back with what, in physics, is called the “normal force.” If you want to go a bit deeper into it, you can talk to students about what is sometimes called the “illusion of solidity,” as follows:
Consider pushing down on a table with your hand. The table will feel solid, but in fact, it is almost entirely made of empty space. To understand why, remember that nearly all the mass of the table is contained in the nuclei of its atoms, while the volume of each atom (essentially the volume occupied by the electrons) is more than a trillion times the volume of its nucleus. This means that the nuclei of adjacent atoms are nowhere near to touching one another, which is why we say the table is almost entirely made of empty space. If we could somehow pack all the table’s nuclei together, the table’s mass would fit into a microscopic speck. (Note: Although we cannot pack matter together in this way, nature can and does—in the strange objects called neutron stars, in which a mass greater than that of the Sun is compressed down to a ball just a few kilometers across.)
So why does the table feel solid? The solidity comes from the way the atoms push back, just as described for the ground in the box. More specifically, this push back comes from a combination of electrical interactions between the charged particles in its atoms and the strange quantum laws governing the behavior of electrons.