# Friction and the third law of motion

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1. Aug 9, 2015

### faiziqb12

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?

2. Aug 9, 2015

### A.T.

At rest realitve to another object providing friction.

3. Aug 9, 2015

### faiziqb12

but why is the force applied by the object rebounded by the surface and not absorbed by it

4. Aug 9, 2015

### HallsofIvy

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.

But as I said this "equal and opposite force is NOT "friction".

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.

5. Aug 9, 2015

### Dr. Courtney

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.

6. Aug 9, 2015

### haruspex

Not as friction, as a normal force.
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.

7. Aug 9, 2015

### Staff: Mentor

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.

8. Aug 9, 2015

### faiziqb12

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

9. Aug 9, 2015

### HallsofIvy

You yourself quoted the "third law"- that is why.

10. Aug 9, 2015

### faiziqb12

thanks!!
thats all what i needed

11. Aug 9, 2015

### A.T.

12. Aug 9, 2015

### Chandra Prayaga

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.