# Problem with Action-Reaction pairs and gravity [duplicate]

There are lot of question on this site similar to this, but they give contradicting opinions.

When standing on earth, my gravitational pull to the earth and earth's gravitational pull towards me cancel, and they are action reaction pairs. Also, I exert a normal force on the ground, and the ground exerts a reactionary normal force, which cancel each other. If this is correct, how is normal force exerted? Should we consider it as another force independent of gravity?

This comes from here.

The normal force is a different force that gravity.

• Just think about when you are skydiving. No normal forces. But the gravitational force in you and your gravitational force in the earth are present.

• Or think about when you are leaning on a wall. Your horizontal push on the wall causes the wall to exert a horizontal normal force on you to hold back. This is not even in direction with gravity.

In general, normal forces and gravitational forces are two entirely different and unrelated things. They just happen to be equal, when an object is at rest on the ground with no other forces acting.

The normal force appears when a material prevents itself from being broken. The table exerts a normal force on an apple, since if it didn't, the apple would break through the table. The normal force is a material-caused "holding back" force.

• A further clarification, how is the normal force exerted? Is it solely due to atomic interactions? – Sheldon Cooper Jun 13 at 13:53
• @SheldonCooper Yes. Normal forces arrive due to the electromagnetic attraction forces that hold atoms, molecules, matter and materials together. They prevent being broken apart by resisting the incoming force. – Steeven Jun 13 at 16:36

There are two equal and opposite pairs of forces in your question. You can identify them by swapping words.

1. gravity: "The earth pulls you" (swap words) "you pull the earth"

2. contact force: "the ground pushes you", "you push the ground"

Then each object can remain stationary as follows

You: the gravity of the earth is pulling you one way, but is balanced by the upward contact force from the ground. Note, this is not an equal and opposite pair in the sense of Newton's 3rd law, those pairs are labelled 1) and 2) above - but the forces are equal and means there is no resultant accelerating force on you.

The earth: the earth is attracted towards you by gravity but also pushed the other way by the contact force, again the forces cancel, but they don't count as a Newton's 3rd law pair.

In a Newton's 3rd law pair, the two forces act on different objects.

The normal force is different to gravity, it is caused by repulsive forces, between atoms, near the surface of each object, if we try and force them too close together.

The other answers are fine, but there is one further point that is worth making.

You say “my gravitational pull to the earth and earth's gravitational pull towards me cancel”. It would be better to say that this pair of forces are equal in magnitude and opposite in direction. They only cancel if you are considering you and the earth together as a single system - in which case the pair of gravitational forces (and the pair of normal forces too) are internal forces and can be cancelled.

But if you consider yourself as one system and the earth as another system then the pair of gravitation forces cannot be cancelled because only one out of the pair acts on you. Similarly, one of the pair of normal forces acts on you.

Now if you are in equilibrium the net force on you must be zero, so we can conclude that the normal force on you is equal and opposite to the gravitational force on you (as long as no other forces act on you). But if you are not in equilibrium (if you are in free fall or doing a push up or in a car going around a banked curve etc.) then the normal force (if there is one) and gravity are no longer equal and opposite.