Friction and the third law of motion

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Discussion Overview

The discussion revolves around the concepts of friction and Newton's third law of motion, exploring the nature of forces exerted by objects on surfaces and the responses of those surfaces. Participants examine the implications of these forces in various scenarios, including objects at rest and the behavior of surfaces under applied forces. The conversation includes theoretical considerations and practical examples.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • Some participants assert that the weight of an object produces a downward force on a surface, which is met with an equal upward force, but they question the nature of this upward force as friction.
  • Others clarify that the upward force is a normal force, not friction, and that friction only arises under specific conditions, such as when there is a sideways force acting on the object.
  • A participant challenges the notion of a "natural tendency" for objects to remain at rest, suggesting instead that objects tend to maintain their state of motion unless acted upon by an external force.
  • Some participants discuss the atomic interactions at surfaces, explaining that friction arises from intermolecular forces resisting motion, rather than being a simple reflection of forces.
  • Questions are raised about why forces applied to an object do not get absorbed but are instead reflected back, with some participants seeking clarification on the terminology used in describing these interactions.

Areas of Agreement / Disagreement

Participants express differing views on the nature of forces and friction, with no consensus reached on the definitions and implications of these concepts. The discussion remains unresolved regarding the characterization of forces as absorbed or reflected.

Contextual Notes

There are limitations in the definitions and terminology used by participants, leading to confusion about the concepts of force absorption and reflection. The discussion also highlights the complexity of friction as a phenomenon arising from atomic interactions.

faiziqb12
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we know weight of an object produces a force on the surface of anything on which it is .. but why the surface gives an equal amount of force back to it as friction ..
the reason is the third law of motion which states every force produces an equal and opposite amount of force ..
so the net force is zero ..
another example is like a football strikes someones leg but often instead of making the leg move it gets back the force applied to it ..
so is the natural tendency o all objects to remain in rest ..
why or why not?
 
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faiziqb12 said:
so is the natural tendency o all objects to remain in rest ..
At rest realitve to another object providing friction.
 
A.T. said:
At rest realitve to another object providing friction
but why is the force applied by the object rebounded by the surface and not absorbed by it
 
faiziqb12 said:

Homework Statement


we know weight of an object produces a force on the surface of anything on which it is .. but why the surface gives an equal amount of force back to it as friction ..
This is not true. If an object is sitting on a surface, the object exerts a (gravitational) force downward on the surface and the surface exerts an equal force upward. But that upward force is NOT "friction". Even a perfectly smooth force will exert that upward force. A surface that is not perfectly smooth will have a "static friction force" that prevents the object from moving but that force is "exerted" only if there is some sideways force acting on the object. If the object is moving on the surface there will be a moving friction that opposes the motion of the object.

the reason is the third law of motion which states every force produces an equal and opposite amount of force ..
so the net force is zero ..
But as I said this "equal and opposite force is NOT "friction".

another example is like a football strikes someones leg but often instead of making the leg move it gets back the force applied to it ..
so is the natural tendency o all objects to remain in rest ..
There is NO "natural tendency" for all objects to remain at rest. There IS a "natural tendency" for all objects to remain in the same motion- so if an object is moving with a given velocity, it will remain moving with that velocity unless there is a force changing it- such as friction.

why or why not?

Homework Equations


all what you may need is f = ma

The Attempt at a Solution



im empty...please help
What exactly is your question?
 
On a flat surface, the normal force (not friction) is equal to gravity.

The frictional force only happens to be equal (and horizontal, not opposite) if the coefficient of friction is 1.0 AND a horizontal force equal to the weight is applied.
 
faiziqb12 said:
we know weight of an object produces a force on the surface of anything on which it is .. but why the surface gives an equal amount of force back to it as friction ..
Not as friction, as a normal force.
faiziqb12 said:
so is the natural tendency o all objects to remain in rest ..
Not at rest, exactly. If no net force acts on a body it does not accelerate. It will continue at the same speed in a straight line. (It may also spin about its mass centre.)
Suppose you place an object weight W on a surface that supports its weight. Because it succeeds in supporting the weight, it must be exerting a force equal to W upwards on the object. Suppose instead we place it on a surface that only supports weight Y < W, a snowpack say. The object will accelerate downwards through the snow. As a result, the object is only exerting a force Y on the snow, so the two forces are still equal.
 
If you zoom in on two surfaces in contact, you will eventually see individual atoms bound in crystal or non-crystal structures. Moving one of the objects makes these atoms push on others and distort the structures. Inter-molecular forces resist those distortions. Therefore it takes force to make them slide. It takes no additional force to just let them sit where they were.

Add that up over trillions of atoms in different arrangements, then take the average, and you have what we call friction. Friction is a highly simplified averaged behavior of very complex things.

Say it another way. If you reduce your two objects to just one atom each, then there is no such thing as friction. There remains primarily electromagnetic forces.
 
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m
HallsofIvy said:
. What exactly is your question?
my question is why does the force given by an object not get absorbed by the object on which the force is acting
but rather gets gets reflected back to the object from which it arised
 
You yourself quoted the "third law"- that is why.
 
  • #10
anorlunda said:
If you zoom in on two surfaces in contact, you will eventually see individual atoms bound in crystal or non-crystal structures. Moving one of the objects makes these atoms push on others and distort the structures. Inter-molecular forces resist those distortions. Therefore it takes force to make them slide. It takes no additional force to just let them sit where they were.

Add that up over trillions of atoms in different arrangements, then take the average, and you have what we call friction. Friction is a highly simplified averaged behavior of very complex things.

Say it another way. If you reduce your two objects to just one atom each, then there is no such thing as friction. There remains primarily electromagnetic forces.
thanks!
thats all what i needed
 
  • #12
faiziqb12 said:
m

my question is why does the force given by an object not get absorbed by the object on which the force is acting
but rather gets gets reflected back to the object from which it arised
Most of the confusion here is because of using words without carefully defining them. What do you mean by a force being absorbed, or reflected? If there is a force (unbalanced) acting on an object, the object accelerates. It neither absorbs nor reflects the force.
 

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