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Normal force Objects really never touch



What are all atoms made of?

In Chemistry we say that atoms can touch each other – they can even bond together


But let’s look more closely at two ways to show this:

The model on the right clearly shows solid-looking atoms touching. But we draw them this only because it is easy to do so.

A slightly more accurate diagram is on the left – clearly showing that the nuclei of these atoms never touch. And if you look carefully you will see that the electrons of each other never quite touch each other.

The model on the left is only a rough approximation – electrons are not really tiny spheres. Under some circumstances they can behave like particles, but under other circumstances they behave like clouds of energy. When you add light, you add energy to these electrons, and their energy clouds can take different shapes, like this:

Clearly, in real life, electrons are not solid objects that whizz around the nucleus, like we wish to imagine, and these clouds do not ever quite touch each other. There is a quantum mechanical force that prevents this from happening, in any circumstance, outside of something extraordinary, like a nuclear fission or nuclear fusion explosion.

So this is what really happens:

Atoms repelling normal force

This summarizes the whole story.

So what does this mean? We think of atoms as solid tiny pieces, and that they can touch each other. It is a useful approximation, and lets us do chemistry problems, but when you get really close to an atom, that model breaks down. When we get really close, we need to use mathematical models which reveal that (a) electrons are clouds of energy that act like waves – they follow special rules, and (b) two electrons never touch.

Depending on how much energy an electron has, a single electron can take any of these orbital shapes:

None of this is to imply that atoms are not real. But we are saying that atoms are not what you think they are. They are mostly made of empty space. And even the sub-atomic particles within these atoms are not solid pieces, but rather, are clouds of energy that follow clearly defined mathematical rules. And these rules tell us that under certain circumstabnces, they can bond together as groups, forming atoms and molecules. Today we can even image molecules directly!  But even with these images, we can see that there is some tiny space between atoms.

First-ever high-resolution images of a molecule as it breaks and reforms chemical bonds, May 20, 2013, Phys.Org


The normal force

In physics, the word “normal” means “perpendicular”.

“The normal force is the support force exerted upon an object that is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. On occasions, a normal force is exerted horizontally between two objects that are in contact with each other. For instance, if a person leans against a wall, the wall pushes horizontally on the person.” – Physicsclassroom.com

The normal force is a special case of the electrostatic force. Namely, solids retain their shape due to the arrangement of the atoms and molecules that make up the solids. The atoms and molecules stay in a particular arrangement due to the electrostatic force that holds atoms and molecules together. The normal force is the force that resists the rearrangement of these molecules. (If there were no normal force, you would fall right through the floor.)

Since the normal force is perpendicular to the surface (as mentioned previously), when you stand on flat ground the normal force acting on you is directed up. However, if you stand on an incline, then the normal would be perpendicular to the incline instead of up. Or if you lean against a wall, you would exert a normal force on the wall, and the wall would exert a normal force on you, with directions that are equal and opposite.


Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework
HS-PS2-1. Analyze data to support the claim that Newton’s second law of motion is a
mathematical model describing change in motion (the acceleration) of objects when
acted on by a net force.

HS-PS2-10(MA). Use free-body force diagrams, algebraic expressions, and Newton’s laws of motion to predict changes to velocity and acceleration for an object moving in one dimension in various situations

A FRAMEWORK FOR K-12 SCIENCE EDUCATION: Practices, Crosscutting Concepts, and Core Ideas
How can one predict an object’s continued motion, changes in motion, or stability?

Interactions of an object with another object can be explained and predicted using the concept of forces, which can cause a change in motion of one or both of the interacting objects… At the macroscale, the motion of an object subject to forces is governed by Newton’s second law of motion… An understanding of the forces between objects is important for describing how their motions change, as well as for predicting stability or instability in systems at any scale.

Massachusetts Science and Technology/Engineering Curriculum Framework (2006)

1. Motion and Forces. Central Concept: Newton’s laws of motion and gravitation describe and predict the motion of most objects.

1.4 Interpret and apply Newton’s three laws of motion.
1.5 Use a free-body force diagram to show forces acting on a system consisting of a pair of
interacting objects. For a diagram with only co-linear forces, determine the net force acting on a system and between the objects.
1.6 Distinguish qualitatively between static and kinetic friction, and describe their effects on the motion of objects.
1.7 Describe Newton’s law of universal gravitation in terms of the attraction between two objects, their masses, and the distance between them.
1.8 Describe conceptually the forces involved in circular motion.

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