# Where's the action force if the normal force is a reaction force?

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• UNForces_885
In summary, the normal force is not a reaction force to the gravitational force because the gravitational force is an action force from Earth on the object, and the normal force is simply the reaction force from the object on Earth according to Newton's third law. This is also true in cases where the normal force is not equal to the force of gravity, such as when the object is on an inclined plane. Some textbooks may refer to the normal force as a "normal reaction force", but this is incorrect as both the normal force and the gravitational force are part of an action-reaction pair. Additionally, the weight vector should originate from the center of mass of the object rather than the centroid of the contact area.
UNForces_885
I know that normal force is not a reaction force to the gravitational force;
- First, because the gravitational force is from Earth on the object (action force) thus the reaction of force is simply form the object on Earth according to Newton's third law.
- Also because in certain situations like when the object is on an inclined plane, the normal force doesn't even equal to the force of gravity on the object but only the perpendicular component of gravitational force.
- My question is: In some textbooks (attached is a screen of these), normal force is named "normal reaction force"; why is that and where's the action force if the normal force is a reaction force?

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W?

Cheers

The word "Reaction" is unnecessary. The normal force is a force exerted by a surface on a body in contact with the surface. The body also (III law) exerts an equal force on the surface, in the opposite direction. These two are an action - reaction pair. Labeling one of them as "Action" and the other as "Reaction", is wrong.

Chandra Prayaga said:
Labeling one of them as "Action" and the other as "Reaction", is wrong.
Or rather meaningless and confusing.

cosmik debris said:
W?

Cheers

Weight

gmax137 said:
Weight
In which case, at least 3 out of the 5 cases shown on the drawing are presented incorrectly. The weight vector is drawn originating from the centroid of the contact area rather than the center of mass of the object.

i.e. There is a difference between your weight (the downforce of gravity on your body) and the downward component of the contact force of your feet on the floor.

gmax137
Good point

## 1. Where does the action force originate from if the normal force is a reaction force?

The action force in this case originates from an external force acting on the object. This external force could be gravity, friction, or any other force that is causing the object to experience a normal force as a reaction.

## 2. How does the normal force act as a reaction force?

The normal force acts as a reaction force by exerting an equal and opposite force on the object that is pushing against it. This is known as Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction.

## 3. Can the normal force ever be the action force?

No, the normal force can never be the action force. As stated in Newton's Third Law of Motion, every action must have an equal and opposite reaction. Therefore, the normal force can only act as a reaction force to an external force acting on the object.

## 4. How is the normal force related to the action force?

The normal force is directly related to the action force in that they are equal in magnitude but opposite in direction. This means that the normal force will always be equal to the external force pushing against it, but in the opposite direction.

## 5. Why is it important to understand the relationship between the normal force and the action force?

Understanding the relationship between the normal force and the action force is crucial in understanding the overall forces acting on an object. It allows scientists to accurately calculate and predict the motion of an object and analyze the forces at play in a given situation.

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