# Question about Newton's third law.

Discussion in 'Science and Nature' started by rawrzzzz, May 4, 2011.

1. What is the equal and opposite reaction of gravity?

2. there isnt an opposite reaction to gravity since every object in the universe has its own gravitational field.

3. Ah. That's kind of what I thought, but I wasn't sure.

4. It's called the Normal force. Usually it's the weight * 9.81 or weight / 32.2 depending on which system you are using. key word being usually here before someone goes into more detail.

5. There is actually antigravity. It is the opposite of gravity. Scientists think there is an opposite of everything like antimatter and matter. It is pretty accepted in science community that there is such thing as antigravity.

6. This is false... EVERY action has an equal and opposite reaction.

The newton's 3rd law pair to Earth's grav. pull on you is the Normal force. The normal force is what prevents you from falling through what you're standing on.

Say you are of mass m = 80 kg (~180lb). Your weight is the force F the earth (g = 9.81 m/s^2) exerts on you:
Code:
`F = mg = 80 * 9.81 =~ 800 Newtons(N) `
Note g is an acceleration, and the equation is analogous F = ma.

Newton's 3rd Law says there is an equal yet opposite force of magnitude. Lets call the normal force N.

Code:
`N = F = mg = 800 N`
So why don't we perceive the earth to 'feel' this force we exert upon it?

Take two sets of the equation F = ma. F1 is for the earth, F2 is for us.

Since both forces are equal (and opposite, but the sign convention is disregarded depending on the reference frame you're looking at), we have a big acceleration and little mass, while the earth has a big mass and a little acceleration.

Code:
```earth
F1 = 800 N = Ma
person
F2 = 800 N = mA
```
Anti-gravity is interesting, but not what the OP was asking for.

7. Antigravity? Seriously?

It's the normal force.

8. somewhat on the same subject, I just don't really wanna start a new thread for this.

E=mc2, Mass x speed of light.. to find the energy what units to you use for mass to multiply the speed of light? just grams?

9. it would be in Newton's. grams is a weight (which is really just a force)

Newton = (kg * m )/ s^2

10. for the mass in E=mc^2, the standard SI unit for mass is Kg, so i;d use that. however you can use any weight, it just means your calculated energy will be a weird figure that you'd have to then convert back to SI units.

11. E (J) = mc^2 (kg * m^2 / s^2)

1 Joule = 1 kilogram * 1 (meter squared per second squared)

12. Thanks alot for the help ya guys, any links to real good videos or websites where I can just dive into this kind of information?

13. Some folks have said that it is the normal force. The normal force is a contact force between you and the earth and it decreases as your angle of elevation increases, which shows that it can't be an equal and opposite reaction to gravity. Newton's law of gravity is F=GMm/r^2. This can be used to determine the earth's gravity acting on you and your gravity acting on the earth. Both of those will yield the same magnitude for the force of gravity, and both forces are an action reaction pair.

14. And so why does your weight not decrease with elevation? The gravitational field will decrease with radius, therefore your weight and normal force as well. It should be noted that the normal force only arises when the body being accelerated by earth's gravity is actually in contact with the ground.

For a falling body though, the only reaction force is the one you stated, the pull of the falling body on the earth.

With Newton's law of Gravity:
Code:
`F = -GMm/r^2;  G = 6.67 x 10^-11`

The mutual force of attraction between yourself and the earth is the same for each object. The earth has a larger mass so it is hardly effected in the same way that you are by this force. This is one reaction pair.

If, on the other hand, you are standing with weight m*g, the reaction pair is the weight contact force, normal force pair. You are applying a force against the ground, and the electrons in your feet and the ground repel each other with equal magnitude of force (m*g) to bring you to equilibrium.

Here is a very helpful video describing Newton's Third Law and some of its implements in the conservation of momentum:
[ame=http://www.youtube.com/watch?v=PZVhhB12QXI]YouTube - Julius Sumner Miller - Physics - Newton's Third Law pt. 1[/ame]

15. There's a difference between angle of elevation and height from the ground. Let's say that you are on the center of a seesaw. If the seesaw is level with the ground, then the normal force acting upon you is equal in magnitude to your weight. If one end of the seesaw is touching the ground, your angle of elevation increases even though your distance from the ground doesn't and the normal force acting upon you decreases. The normal force is only equal to your weight if your angle of elevation is zero. This is an example of the forces being equal and opposite, but not being an action/reaction pair.

I can't get youtube videos on my computer, but I understand how conservation of momentum come out of Newton's laws.

16. I think you are still confusing the two different occurrences of the 3rd law. The normal force is a contact force; it responds to the push by contact you exert with your weight. This force at a molecular level is roughly a summation of electric forces. There are only 4 forces, Electromagnetism, Gravitation, Electroweak, and Color/Strong forces.

It is not the same reaction pair as your gravitational pull on the earth and the earth's pull on you.

As you increase the angle of elevation with the see-saw, the component of the contact force weight acting parallel (and thus the normal force to counteract it) are slowly reduced as the other component battles friction.

17. The contact force that you exert with your weight is caused by gravity, but it isn't gravity. You are correct, it is electromagnetism. You also got one of the four forces wrong. The first two and the last are correct, but the electroweak force is a force that existed at the early universe when the EM force and the weak nuclear force were indistinguishable.

Correct, therefore the normal force is not the equal and opposite reaction to gravity.

This all happens while the force of gravity remains the same and you are confusing the term parallel with perpendicular. Friction is irrelevant.

When gravity and the normal force are equal and opposite vectors, it's because of Newton's first law, not his third.

18. #18
Last edited by a moderator: Mar 15, 2016
Yep, I've been reading Brian greens new book "A Hidden Reality" and he talks a lot about it. Newton first thought of it with the cosmological constant then threw it out, but today they are finding it could be true. What else would be the opposite of gravity on the scale of galaxies and planets? What would the normal force be in the case? It's antigravity. On the scale of a car staying on earth it's the normal force but I'm talking on a greater scale. This isn't high school physics. This is advanced modern physics.

19. You are confusing Newton with Einstein and you are completely mistaken about the normal force. The normal force is a contact force for an object on a surface. It has nothing to do with shit in space. When planets are gravitationally attracted, the force that each one does to the other is equal an opposite. Anti-gravity is irrelevant.

20. The real answer is gravity upon the other object. If I go skydiving I'm gonna fall a mile or so but THE EARTH will fall up by a small distance.

It cannot be the normal force because ghat only exists while you press upon and object, which would mean in freefall, when you press on nor surface, there is no reaction force.