# Normal force cancels out the force of gravity?

Mark44
Mentor
Can you give an example of a third law partner in regards to a situation when a 10kg object rests on the ground?
Translation of the Latin Newton's 3rd law was written in: (from Wikipedia - http://en.wikipedia.org/wiki/Newton's_laws_of_motion#Newton.27s_3rd_Law ) "Law III: To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts."
The 10 kg object exerts a force on the ground. The partner force would be that the earth is exerting a force on the object. The two forces are equal in magnitude, but oppositely directed.

I don't understand why you're having such a hard time with this.

The 10 kg object exerts a force on the ground. The partner force would be that the earth is exerting a force on the object. The two forces are equal in magnitude, but oppositely directed.
Yeah, I've got no problem with this.
But jbriggs444 said there that in addition to these two forces, there are other "third law partner" forces that act on other objects...
What are those additional "partner" forces?

Mark44
Mentor
Yeah, I've got no problem with this.
But jbriggs444 said there that in addition to these two forces, there are other "third law partner" forces that act on other objects...
What are those additional "partner" forces?
I think he (jbriggs) was talking about the repulsion forces of the earth against the object, and its repulsion against the earth. If those aren't what he was talking about, he can chime in.

jbriggs444
Homework Helper
The ground exerts a contact force on the 10 kg object. The 10 kg object exerts an equal and opposite contact force on the ground.

The earth exerts a gravitational force on the 10 kg object. The 10 kg object exerts an equal and opposite gravitational force on the earth.

The ground exerts a contact force on the 10 kg object. The 10 kg object exerts an equal and opposite contact force on the ground.
When you say "contact force" do you just mean a normal force or normal force + some another force?
Earlier you said:
A "normal" force is the component of the contact force

jbriggs444
Homework Helper
When you say "contact force" do you just mean a normal force or normal force + some another force?
I meant the total contact force -- the normal component plus whatever tangential (aka frictional) component might also exist. I was careful to use the term "contact" rather than the term "normal" just in case we are talking about an object on a sloping surface.

When you say "contact force" do you just mean a normal force or normal force + some another force?
Yes, I already stated this in the previous reply. The whole "normal force" comes from the concept of Newton's third law of motion. Whatever force you apply downward, the ground's going to exert the same and opposite force upward. This upward force is called "normal".

And many people stated more than one force acting here. No, in an isolated situation dealing with the normal, all you bother to count is the forces acting downward.
I gave an example of the table before, refer to that.

Khashishi
It's possible for an object to bounce off the ground, but there is some dissipation whenever some macroscopic object falls onto the ground, so eventually the object will rest in contact with the ground. In the state, the normal force and gravitational force cancel out, and the object stays at rest. Depending on how precise you are trying to get with microscopic physics, you could say the object is floating above the ground by a small distance (on the order of an angstrom) due to electromagnetic and exchange forces in the atoms.

vizakenjack
What's the difference between a contact force and a normal force?

Put really simply, contact force is any force that acts at the point of contact between two objects. The normal force and friction are both instances of contact force.

http://en.wikipedia.org/wiki/Contact_force

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vizakenjack
Alright, so I know there will be a normal force pointing perpendicular to the surface and gravitational force, right?

Gravitational force = mg
But since normal force acts in "opposite direction to the gravitational force". Wouldn't normal force then cancel out the gravitational force? If it's so, wouldn't you just "fly" for a moment up, then come back? You'll come back because once you stop touching the ground, normal force would stop acting on you...

If an object rests on the ground, then angle is zero
cos(0) = 1.

dude you should have aclear notion that an object will only have motion in
a direction only if it has some velocity or acceleration in that direction.we stay on the ground becoz we dont have anyvelocitu or acceleration in the upward direction.when we jumpwe have a velocity in upward direction.at that instant no normal force afting on us and becoz of gravitational force we get pulled back down

Plus forces vary on the basis of your point of view i.e. If you want to considr forces on the man there are only two
1.mg by the earth
2.normal force by the surface
cases where friction comes into play also needs the tendency of an object to move in a particular direction which is opposed by frictional force.

Philip Wood
Gold Member
An interesting question to ask is how a table manages to provide a normal force equal (and opposite) to the weight of any object (within reason) I care to place on it. The answer is that the table compresses/buckles UNTIL the forces balance. It's just like placing a body on a top-pan balance with a very stiff compressible spring inside it. In fact things are a little more complicated than I've made out, because there will be rapid oscillations of imperceptible amplitude if the object is just plonked on the table. The oscillations will rapidly damp out. Hope this helps.

1. Newton's third law is widely misunderstood. His 'action' and 'reaction' were not forces but impulses. "An impressed force is an action to change the motion (momentum) of a body". Newton specifically denied that the equal and opposite gravitational forces between bodies were examples of the third law, If they were not equal this would lead to outcomes that were "absurd and contrary to the first law". The third law is only needed for net forces. 'Equal and opposite forces in an interaction' does not presuppose that anything is accelerating and is required for conservation of energy. Equal and opposite net forces in an accelerating system are required for conservation of momentum.
2. A body at the surface of the Earth and the rest of the Earth interact gravitationally. This means that each experiences a force of the same magnitude, directed towards the other and independent of any other interaction in which either or both may be involved.
3. Because the Earth is not uniform the direction of the gravitational force is not necessarily towards the actual center of the Earth.
4. Because the Earth is rotating, and everything on it is in accelerated motion, other than at the poles, the normal force at a horizontal surface is not equal to the gravitational force on a body at rest on the surface. It is somewhat less. The difference is quite easy to calculate.

Hello guys, I had seen a lot of threads on gravity but they are closed now. I have a curiosity for asking this question.
Instead of defining gravitational force in a rather complex way (like curved space time, etc..) can we define it simply through our observations? Like can we say that gravity is just a net effect of van deer Waals force of all the molecules summed together? It seems sensible to me as it is also somewhat proportional to mass.
Also could it be some net force related to EM force itself, since the sub atomic particles are not stationary within an atom, could that net fluctuations may support gravity somehow?(like in the case of van deer Waals forces)
(Also sorry if I have asked something wrong or silly or unrelated. I am just curious to learn)

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@Shri13: Van Deer Waals forces are in consistent. Like gravity, they are week forces, but unlike gravity, they appear and disappear inconsistently. The force of gravity is always proportional to the inverse square if the distance between the two objects and directly porportional to the pruduct of their masses. There's a gravitational constant, but because it is a constant we can just ignore it for now. In regard to subatomic particals, the centripital force of attraction is due to opposite charges, protons and electrons, their inertia and co-interaction keeping them from crashing inward. Lastly, keep in mind that the four great forces of the universe were initially one super force immediately after the Big Bang, and that it was shortly there after that each if the independent forces, one of which was gravity, split off from one another... And that the entropeic nature of the universe is why gravity was able to pull together the cosmos as it has for 14.5 billion eat.

@Shri13: Van Deer Waals forces are in consistent. Like gravity, they are week forces, but unlike gravity, they appear and disappear inconsistently. The force of gravity is always proportional to the inverse square if the distance between the two objects and directly porportional to the pruduct of their masses. There's a gravitational constant, but because it is a constant we can just ignore it for now. In regard to subatomic particals, the centripital force of attraction is due to opposite charges, protons and electrons, their inertia and co-interaction keeping them from crashing inward. Lastly, keep in mind that the four great forces of the universe were initially one super force immediately after the Big Bang, and that it was shortly there after that each if the independent forces, one of which was gravity, split off from one another... And that the entropeic nature of the universe is why gravity was able to pull together the cosmos as it has for the past 14.5 billion years.

The moon orbit is due to centrifugal force.
An apparent force that acts outward on a body moving around a center, arising from the body's inertia.

A.T.
The moon orbit is due to centrifugal force.
- In the inertial frame of reference there is no centrifugal force on the Moon.

- In the rotating rest frame of the moon, the moon is not orbiting anything, but simply at rest.

So explaining "orbits" with "centrifugal force" doesn't make sense in either frame of reference.

Okay, try this: If the Moon was not racing around the Earth it would succumb to the combined gravitational forces of Earth and Moon and crash.

jbriggs444