# Friction and an object stopping

nav888
Homework Statement:
Explain why the frictional force stops acting on an object once it's velocity equals 0 (if the frictional force has caused the object to decelerate to rest)
Relevant Equations:
F ≤ mu R
Initially I tried to explain it via kinetic energy of the object and how the frictional force can only do as much work on the object as the object has kinetic energy but I got confused. Could someone here please explain why if I have an object with a net force due to friction (and no other forces acting horizontally) the object will slow down (I know this due to newtons first and second law), but why does the friction stop acting and not continue to accelerate the object backward?

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The statement is simply false. Friction can act on an object at rest.

nav888
The statement is simply false. Friction can act on an object at rest.
Let me rephrase the question

If I had a box on rough ground and was able to overcome the static friction and accelerate the box, and then stopped pushing the box, it's obvious that the box would stop moving eventually and come to rest. What I'm asking is why does the kinetic friction which causes the box to decelerate stop acting on the box once the box comes to rest. I know the kinetic friction must stop acting otherwise the box would accelerate back towards me (which makes no sense at all)

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why does the kinetic friction which causes the box to decelerate stop acting
Because friction, kinetic or static, acts to oppose relative motion of the surfaces in contact.

nav888
Because friction, kinetic or static, acts to oppose relative motion of the surfaces in contact.
So if there's no relative motion there's no friction?

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So if there's no relative motion there's no friction?
If there is no tendency to motion there's no friction. If there is no actual motion there may be static friction.

nav888
If there is no tendency to motion there's no friction. If there is no actual motion there may be static friction.
Is it ok to think like this.
The object will dececelrate due to friction then once the relative velocity of the object with respect to the ground become 0 the friction will have "done its job" and no longer act on the object. but we might have static friction if we apply a force and the object doesn't move

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Is it ok to think like this.
The object will dececelrate due to friction then once the relative velocity of the object with respect to the ground become 0 the friction will have "done its job" and no longer act on the object. but we might have static friction if we apply a force and the object doesn't move
or consider a box sliding down a ramp, slowing. It comes to rest even though there is still a gravitational force tending to move it down the ramp. Kinetic friction is replaced by static.

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Explain why the frictional force stops acting on an object once it's velocity equals 0 (if the frictional force has caused the object to decelerate to rest
Symmetry demands that it be so.

If we have an object at rest (relative to the surface on which it sits) then there is no preferred direction based on its velocity. Every direction is the same as every other relative to a zero velocity.

If the object is subject to no other external force (in the horizontal plane) then, again, there is no preferred direction based on the external force.

We assume that there is the mating surfaces where friction exists have no particular preferred direction.

We normally take it for granted that our force laws are based on the current system state. Things such as present position and present velocity. Historical information does not come into it.

But if the current state has no preferred direction then the prediction for force can have no preferred direction. So the only possible definite prediction for the frictional force along the horizontal plane must be the zero vector.

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Because friction, kinetic or static, acts to oppose relative motion of the surfaces in contact.
I don't understand the "or static" part of this statement. Consider a person standing up at rest. At some point he starts walking.
1. His velocity has changed in the forward direction, therefore the acceleration is in the forward direction and so is the net force on him while he accelerates.
2. Two entities act on the person, Earth that generates a force in the vertical direction and the ground surface that generates a force that can be resolved in two components, normal to the surface and parallel to the surface. The latter is static friction (assuming that the soles and surface are at rest relative to each other.)
3. If we remove static friction, the person will not be able to start walking. For example, the same person on rollerblades will not be able to propel himself forward in the same manner as in normal walking. Therefore without static friction there can be no forward motion.

From all of the above, I think it is safe to conclude that static friction in the forward direction provides the propelling force and is in the same direction as the motion of the man relative to the ground. Maybe I missed the point of "opposing relative motion of the surfaces in contact", but it seems to me that when friction is only static, there is no such relative motion by definition.

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Let me rephrase the question

If I had a box on rough ground and was able to overcome the static friction and accelerate the box, and then stopped pushing the box, it's obvious that the box would stop moving eventually and come to rest. What I'm asking is why does the kinetic friction which causes the box to decelerate stop acting on the box once the box comes to rest. I know the kinetic friction must stop acting otherwise the box would accelerate back towards me (which makes no sense at all)
Perhaps we should first differentiate between friction and force of friction.

Friction:
At microscopic level, the surfaces in contact “grab” each other.
Some combination of materials and smoothness of the surfaces tend to grab more than others.
When there is relative motion, that “grabbing” effect is less strong than when the surfaces tend to “clamp” together.

Force of friction:
This is always a reaction type of force.
Because of that, it mimics the action type of force that is applied onto the direction parallel or tangential to both surfaces.
The magnitude of both, action and raction force, increases from zero to certain value without being able to induce any movement (zero increment of kinetic energy); therefore the “static” name.

When relative motion is initiated and maintained, again both forces (pushing or pulling and kinetic friction) keep acting in opposite direction and with equal magnitudes.
The pushing or pulling force gives energy or momentum to the body, while the kinetic friction force always consume all or part of that energy from the body.

If the motion inducting force is removed, only the energy consuming friction force remains, but only until that energy is fully depleted and the body stops.

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Hi @nav888. If you haven't yet got your head around it, maybe this video (despite its flaws) might help...