The Forces On You (Introducing Normal Force)

What forces (i.e. pushes or pulls) are acting on you right now? Most people can identify the gravitational force down, but there must be something else otherwise you would accelerate down towards the center of the Earth. The other main force on you is called the normal force. It is a force perpendicular to the surface that supports you, like the ground or the seat of your chair. You compress this surface and it acts like a spring, pushing you up.

Category:

Education

Tags:

• force
• weight
• normal force
• gravitational force
• force of gravity
• free body diagram

[MrLatrommi]

Good video. However, usually when we combine the normal reaction forces acting on a person's leg from the ground, we still draw this combined force's point of application from the ground and not from the centre of gravity of the person. You may also want to take note of the length of the vectors since they represent the magnitude of the forces.

[1veritasium]

@MrLatrommi Most introductory physics courses draw all forces acting through the centre of mass because this avoids complexities due to rotational motion.

[bramshih]

You gave the example of standing on a spring or the ground, but what about standing on a weak wooden table? Eventually, the table would no longer be able to hold you up and would break. So my question is: are hardness and density (if those are even the right term) factors in the amount of normal force? If so, wouldn't this contradict with Newton's 3rd law because you can have "less normal force" than gravitational force?

[1veritasium]

@bramshih This is a good question. The answer is Newton's Third Law is never violated. If you are standing still the normal force must be equal to your weight. If a table breaks beneath you, then the normal force is less than weight and you accelerate to the Earth. This doesn't violate Newton's third law because the equal and opposite reaction force is actually the force with which you attract the earth. Check out my vid on Newton's third.

[nubbs]

what about the force acting on the atoms in your feet and the ground that keep you from falling through the ground?

[1veritasium]

@nubbs haha, yes that's important - that's what the analogy with the spring is meant to be. That reminds me of the story "turtles all the way down." Check it out on wikipedia.

[tacticalstation]

Good job, you just clearly explained what most college physics professors attempt in 40 minutes to get across to their students.

[Darrin1997]

What about atmospheric force?

[nafativedec]

In a more detailed way - The body and the ground are made of atoms, and the atoms have electrons which have negative charge. As we all now the same charge repel, so the electrons in the ground repel the electrons in your feet. It's due to the electromagnetic force that we don't fall trough the ground in the center of the earth.

[ClassicJustice]

Great videos; honestly, but who laughed at 2:06?

[naruto4ever66]

so is force normal just an application of newton's third law? when gravity pulls you down towards the earth, you are also exerting a force on the surface below you, and the surface reacts with an equal and opposite force on you?

[mrsg9064]

Greetings from San Diego, CA. Wanted to let you know that we are using your videos in my 8th grade science class. They are terrific!! The students really enjoy your style of "teaching". Thanks so much!

[DoomVik]

So... do you know why does atoms attract each-other and pushing away each-other?

[MrLatrommi]

@1veritasium Yes. It is quite obvious that the net moments about the c.g. of an object is zero if it is in equilibrium. Nonetheless, in this case, by drawing the combined reaction force from the ground acting on a person such that its line of action coincides with its weight, the principle of conservation of moments applies. A little cautiousness goes a long way, especially in education.

[OfficialBrizzyBeats]

2:06 LOL c;

[T3AI2S]

2:06

lolololololololololololol

look at the picture