• B
• Sundown444
In summary, there can be multiple action-reaction forces between two objects, such as a chair and the floor, including friction, normal force, and applied force. These forces can be resolved into components, but they are still considered one contact force between the two objects. The distinction between "action" and "reaction" forces is determined by the chosen system in a free body diagram, but they are ultimately symmetrical in nature.
Sundown444
I have a question about action-reaction forces, and it mostly has to do with contact action and reaction forces, but it can include action-at-a-distance forces, too. So, can there be multiple action-reaction forces on the same two objects, like a chair and the floor, for example? By multiple action-reaction forces, I mean those of normal force, applied force, friction force, drag, etc.

Sundown444 said:
I have a question about action-reaction forces, and it mostly has to do with contact action and reaction forces, but it can include action-at-a-distance forces, too. So, can there be multiple action-reaction forces on the same two objects, like a chair and the floor, for example? By multiple action-reaction forces, I mean those of normal force, applied force, friction force, drag, etc.
Yes, every force acting on an object will separately have its own 3rd law counterpart on a different object. So if a chair has the normal force from the floor, its weight from gravity, and a contact force with someone's butt then there will be three third law forces. One will act on the floor, one will act on the earth, and one will act on the butt.

vanhees71 and Sundown444
Dale said:
Yes, every force acting on an object will separately have its own 3rd law counterpart on a different object. So if a chair has the normal force from the floor, its weight from gravity, and a contact force with someone's butt then there will be three third law forces. One will act on the floor, one will act on the earth, and one will act on the butt.

I see. I probably misspoke here, but what I meant would there be more action-reaction forces acting between just the chair and the floor? Like friction, normal force and applied force all acting as separate action-reaction forces between the chair and the floor? This does not include a person sitting on the chair, by the way.

Sundown444 said:
Like friction, normal force and applied force all acting as separate action-reaction forces between the chair and the floor?
Sure, you could have that. Suppose that the chair is magnetic and the floor is metal. Suppose further that the chair is being pushed from the side. Then you would have three forces from the floor acting on the chair: the magnetic force, the normal force, and the friction force*. Each would have its own 3rd law counterpart acting on the floor.

*In principle you could say that the normal force and the friction force are just different components of the same contact interaction so they together really form one force. However, you still have two forces with the combined contact force and the magnetic force.

Sundown444
Sundown444 said:
I see. I probably misspoke here, but what I meant would there be more action-reaction forces acting between just the chair and the floor? Like friction, normal force and applied force all acting as separate action-reaction forces between the chair and the floor? This does not include a person sitting on the chair, by the way.
Normally, one would add vectorially all the forces exerted by one entity on another and call that the action force. The reaction force exerted by the other on the first would be equal in magnitude and opposite in direction to the first force. Now this doesn't mean that the single force exerted by the entity cannot be broken down into components. For example, if the chair is dragged across the floor, there is a single force exerted by the floor on the chair. That force is usually resolved into two components: a component perpendicular to the floor which given the name "normal force" and a component parallel to the floor which is called "force of kinetic friction". The point here is, one entity - one contact force with another entity. Obviously, you can resolve that one contact force into as many components as you like and give them separate names, but this does not make them separate forces. If nobody is sitting on the chair and the chair is not dragged across the floor, then the contact force exerted by the floor on the chair is pependicular to the floor, i.e. there is no parallel component.

Dale
kuruman said:
Normally, one would add vectorially all the forces exerted by one entity on another and call that the action force. The reaction force exerted by the other on the first would be equal in magnitude and opposite in direction to the first force.
I would resist the urge to call one sum the "action force" and the other sum the "reaction force". There is nothing special about an "action force" that distinguishes it from a "reaction force". Neither is cause. Neither is effect. They simply coexist. Forces come in pairs.

I would also resist the urge to lump the force from the floor onto the three legs of a three legged stool into one force and then combine that with any magnetic force that might also exist. It might be useful to summarize those forces. Or it might not. Choosing to label the sum as the "action force" would be irrelevant either way.

sophiecentaur
jbriggs444 said:
I would resist the urge to call one sum the "action force" and the other sum the "reaction force". There is nothing special about an "action force" that distinguishes it from a "reaction force". Neither is cause. Neither is effect. They simply coexist. Forces come in pairs.
Right. However, I think that what distinguishes the "action" from the "reaction" forces is the choice of system in a FBD. The action forces are the ones that are drawn in it, act on the system and are added to give the net force that causes the acceleration of the system. By contrast, their reaction counterparts are out of sight but not out of mind. The inherent symmetry that you mention is broken when one chooses a system.
jbriggs444 said:
I would also resist the urge to lump the force from the floor onto the three legs of a three legged stool into one force and then combine that with any magnetic force that might also exist. It might be useful to summarize those forces. Or it might not. Choosing to label the sum as the "action force" would be irrelevant either way.
Of course. I oversimplified and I should have said "one contact force per leg". I also did not mention that the point of application of these forces is relevant because there might be torques involved.

jbriggs444
kuruman said:
However, I think that what distinguishes the "action" from the "reaction" forces is the choice of system in a FBD. The action forces are the ones that are drawn in it, act on the system and are added to give the net force that causes the acceleration of the system.
You can choose to call them reaction forces, if you like.

Mister T said:
You can choose to call them reaction forces, if you like.
Sure. I could also choose to call them Fred or Larry. I chose "action" because it's easier conceptually to think of them as such, especially for FBD practitioners. Most, if not all, FBD recipes start with something like "Define your system and draw all the forces acting on it." In other words, "Of all the pairs of forces in the Universe, pick the forces that contribute to changing the velocity of the system and draw them." This last statement can be made shorter and in agreement with everyday language usage if two words "action forces" replace the locution "forces that contribute to changing the velocity of the system."

In cases where a system has more than one components, e.g. two stacked blocks accelerated as one by a single force exerted on the bottom block, static friction is the action (and net) force on the top block. Static friction is also an action force on the bottom block, its reaction counterpart being the action force on the top block. So choosing what to call action and what to call reaction has meaning only relative to an already chosen system. Without a system, we might as well use Fred and Larry.

## 1. What is an action-reaction force?

An action-reaction force, also known as Newton's third law of motion, states that for every action, there is an equal and opposite reaction. This means that when an object exerts a force on another object, the second object exerts an equal and opposite force back on the first object.

## 2. How does an action-reaction force affect motion?

An action-reaction force does not affect the motion of an object. This is because the action and reaction forces act on different objects and therefore do not cancel each other out. They simply act in opposite directions.

## 3. Can an object exert an action-reaction force on itself?

No, an object cannot exert an action-reaction force on itself. This is because the action and reaction forces must act on different objects in order to satisfy Newton's third law of motion.

## 4. Are action-reaction forces always equal in magnitude?

Yes, according to Newton's third law of motion, action and reaction forces are always equal in magnitude. This means that if one object exerts a force of 5 Newtons on another object, the second object will exert a force of 5 Newtons back on the first object.

## 5. How do action-reaction forces apply to everyday situations?

Action-reaction forces can be observed in many everyday situations. For example, when you push against a wall, the wall pushes back on you with an equal and opposite force. Another example is when a rocket launches into space, the force of the rocket pushing down on the ground is matched by the reaction force of the ground pushing up on the rocket.

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