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## Physics library

### Course: Physics library>Unit 3

Lesson 2: Normal force and contact force

# Normal force and contact force

The force that keeps a block of ice from falling towards the center of the earth. Created by Sal Khan.

## Want to join the conversation?

• If you stripped all of the electrons from an atom, does that mean it could pass through matter? • I'm confused. It sounds like 'normal' force and 'contact' force describe the same thing. If I understand correctly, 'contact' force is happening on an atomic level between the object and the surface, which results both in friction and not allowing the object to pass through the surface. If the contact force is responsible for not allowing the object to pass through the surface, then isn't the normal force part of the contact force? Or is it something completely separate? •   At around , Sal begins to explain that there needs to be a force to oppose the weight of the ice cube. At , he calls that the normal force. The normal force is a force perpendicular to the ground that opposes the downward force of the weight of the object. That's all there is to it and you don't have to think of it in terms of individual atoms in most problems you come across.

The 'normal' force is a type of 'contact' force. What Sal doesn't clarify in this video is that the contact force is ANY force that results when two things (and their atoms and molecules) touch each other. It's a general term that can refer to normal force, friction, collision force, tension, etc. For example, if you throw a ball at a wall and it hits the wall at an acute angle, the force at the angle of contact on the ball is a contact force. Don't confuse this force with normal force. Remember the term "normal force" specifically refers to a force that's perpendicular to a surface. So if a contact force is, for example, at a 45 degree angle, it can't be considered a normal force.
• what is a situation where no normal force is acting upon a person. • well, it depends:
1. If you are on earth´s surface you´ll just pass through earth till you´ll get fused into earth mantel and PAM PAM PAM you are one with the earth and you the earth is one with you.
2. If you are in deep space on a platform and suddently there is no normal force then if we supposely tell that there are no other forces applying on you then you´ll just float around because there is no gravity net force that is strong enough to make you go through the platform so yeah in both case it wouldn´t end so well for you...
• Does Newtons 3rd Law apply in deep space vaccum? • So, normal force would be the "reaction" of the force of gravity? I don't get it • The word "Normal" really just means "perpendicular to a surface". The "normal force", as you are using the term, is referring to the force that a surface, like a table, would exert on an object, like a box, to support its weight (the "weight" is the force an object exerts due to the acceleration of gravity). If the table is level, that force is equal and opposite to the weight of the box, and so is the only "reaction" to gravity.
If the table is not level, the "normal force" will no longer be directly opposite gravity. In a simple static problem, the normal force will be equal to the component of the weight which is perpendicular to the table. The remaining weight will need to be supported by the addition of friction or an applied force.
• what happens when the constant moving block gets tilted to the left and hits the other block?
will the stationary block go as fast as the other block goes on impact? • Let's just take out the tilt and say the block is moving at 5 m/s to the left, on a collision course with the stationary block. Now think of Newton's 3rd law of motion: Every action has an equal and opposite reaction. The force will be divided between the two blocks and they will both move away from each other at 2.5 m/s assuming no energy lost in the collision.
• The electrostatic force between to molecules makes them repel each other and this force gets stronger the closer they are to each other so i guess this means they never actually come into contact. Does this mean we never actually 'touch' anything or do the particles eventually come into contact? • wow; cool question.

My answer is this; 'it depends on how you define touch'. and I encourage you to have a go at producing your own definition
This also raises the question of how BIG the atom is. ie how do you define 'size'?

Remember, you are talking about the repulsion between electrons here right? The outer shells of the molecules fixed in the two materials.
if we push two protons together....at what point would they 'touch' in your definition?

Now if we keep pushing the protons until they get very close (about a proton width apart) then another force comes into action; the strong nuclear force and this is a) attractive and b) much stronger than the electrostatic force. So the protons can 'stick' together, forming a larger atom. However, this is unstable and neutrons will also be required to help it all stick together.
• If we call Gravity and Normal force Action-Reaction pair then it would be wrong right? Since gravity is not a force exerted by the earth but the object!
Am I correct?   