Why is there no action-reaction pair for normal force?

In summary: But the main thing is, we really need to understand what action/reaction pairs are, what they are for, and how they work, before we can start to use them properly in a situation like this. I don't think you've got that yet, so here's a question:Suppose you are in an elevator that is being lowered down at a constant speed of 2 m/s. You are holding a 10 kg weight up over your head, waiting for the elevator to reach the ground floor so you can give it to your friend. Which force is greater, the force of the weight on your hand, or the force of your hand on the weight?In summary, the normal force and weight are not
  • #1
jlyu002@ucr.e
60
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Here is an example in which the normal force would not be counted as an action-reaction pair. A 100g block is sitting at rest on a horizontal table. According to Newton's third law, which of the following indicates the correct action-reaction pair of the two forces? So I was stuck between C. The weight of the block and the normal force exerted by the table on the block and D. the weight of the block and the gravitational force exerted by the block on Earth. The correct answer in my textbook was D. It's pretty weird. Why is there no action reaction pair for the normal force?
 
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  • #2
this is the action reaction pair of normal force:the block applies a normal force on the table, which is equal to its weight, in response to which the table exerts a normal force on the block of the same magnitude.
 
  • #3
Hi jlyu002@ucr.e! Welcome to PF!

I find that strange too. When the block is falling, say, there is no weight but only the gravitational force. I don't think one part of an action-reaction pair can stay alone.

EDIT: I misunderstood. Doc Al below has the right idea.
 
  • #4
By the way, this is not homework or course work. It is a general question that has been nagging at my mind for months. The example I used is to further supplement my question by guiding the pundit to the right answer.
 
  • #5
jlyu002@ucr.e said:
Here is an example in which the normal force would not be counted as an action-reaction pair. A 100g block is sitting at rest on a horizontal table. According to Newton's third law, which of the following indicates the correct action-reaction pair of the two forces? So I was stuck between C. The weight of the block and the normal force exerted by the table on the block and D. the weight of the block and the gravitational force exerted by the block on Earth. The correct answer in my textbook was D. It's pretty weird. Why is there no action reaction pair for the normal force?
The normal force certainly is part of an 'action/reaction' pair. It's just that that choice wasn't given.

The action/reaction pairs in this situation are:
-the downward gravitational pull of the Earth on the block (its weight) and the equal and opposite upward gravitational pull of the block on the earth
-the upward push of the table on the block (the normal force) and the equal and opposite downward push of the block on the table

The big mistake that most make is thinking that the weight and the normal force are action reaction pairs. They are not. If you remove the table (toss the block in the air), the weight remains but the normal force disappears.
 
  • #6
Hey Doc Al,
The first action-reaction pair made plenty of sense to me. The second part, not so much. This is probably due to my lack of comprehension of the subject. Is this "push," something other than the weight of the block?
 
  • #7
I think I see the problem. The force that the table exerts on the book does not equal the force of the Earth on the book. The force of the Earth on the book is the weight of the book. The forces in an action-reaction pair must act on different object. Hmmm. Yeah, I still cannot understand the second action reaction pair you addressed Doc Al - the upward push of the table on the block (the normal force) and the equal and opposite downward push of the block on the table.
 
  • #8
jlyu002@ucr.e said:
Hey Doc Al,
The first action-reaction pair made plenty of sense to me. The second part, not so much. This is probably due to my lack of comprehension of the subject. Is this "push," something other than the weight of the block?
Remember that 'weight' is the gravitational force exerted by the Earth on the box. The normal force, on the other hand, is the contact force (electromagnetic) between two objects that push against each other.

Under the usual zero-acceleration conditions, the upward normal force on the box will equal the downward weight of the box. But that doesn't make them action/reaction pairs. (For one thing, action/reaction pairs act on different bodies!)
 
  • #9
jlyu002@ucr.e said:
I think I see the problem. The force that the table exerts on the book does not equal the force of the Earth on the book.
They may well be equal and opposite, but that doesn't make them Newton's 3rd law pairs.
The force of the Earth on the book is the weight of the book.
Right.
The forces in an action-reaction pair must act on different object.
Right!
Hmmm. Yeah, I still cannot understand the second action reaction pair you addressed Doc Al - the upward push of the table on the block (the normal force) and the equal and opposite downward push of the block on the table.
See my last post.

But realize that whenever body A pushes on body B, that body B must push back on body A with an equal and opposite force.

So the box and the table push on each other with equal and opposite forces.
 
  • #10
Perhaps it would help to recognize that the "purpose" of action/reaction pairs is not to create a force balance on a given object within a system (as Doc Al said, action/reaction forces don't cancel on the pieces of a system, they only cancel on a whole system that includes all the interacting objects), their purpose is to ensure overall conservation of momentum of the whole system. If the Earth could exert a force of gravity on a book, but the book did not exert the same force on the Earth, then when a book falls, the Earth+book system would not conserve its momentum. Also, when the book hits the table and experiences a normal force, if the book did not also push on the table, then the downward fall of the book would be stopped, while the "upward fall" of the Earth would not be stopped-- so again momentum of the Earth+book system would not be conserved. Maybe that will help.
 
  • #11
Sweet! Thanks you guys. I can sleep better now. 8)
 

1. Why is there no action-reaction pair for normal force?

According to Newton's third law, for every action, there is an equal and opposite reaction. However, normal force is not a force that results from a specific action, but rather a force that arises when two objects are in contact with each other. Therefore, there is no specific action that can be identified as the cause of the normal force, and thus, there is no corresponding reaction force.

2. What is normal force and how is it different from other forces?

Normal force is a force that is exerted by a surface on an object in contact with that surface. It is perpendicular to the surface and its magnitude depends on the weight of the object and the surface it is in contact with. Unlike other forces, such as gravity or friction, normal force is not a force that one object exerts on another, but rather a force that arises from the contact between two objects.

3. Can normal force ever have an action-reaction pair?

No, normal force does not have an action-reaction pair. As mentioned earlier, normal force is not a force that results from a specific action, but rather a force that arises from the contact between two objects. Therefore, there is no specific action that can be identified as the cause of the normal force, and thus, there is no corresponding reaction force.

4. How does normal force affect an object's motion?

Normal force does not affect an object's motion directly. It is a force that balances out other forces acting on an object, such as gravity or applied force, to keep the object in contact with a surface and prevent it from sinking into the surface. In other words, normal force helps maintain the object's equilibrium and stability.

5. Is normal force always present?

No, normal force is not always present. It only arises when two objects are in contact with each other. For example, when you stand on the ground, the normal force of the ground acts on you to support your weight. But if you were to jump, there would be a brief moment when you are not in contact with the ground and there would be no normal force acting on you.

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