# What friction causes objects to decelerate?

• I
Clockclocle
When I exert enough force that overcome the static friction. The object start moving and surface create kinetic friction on object if I exert harder overcome the maximum of friction it start accelerate. When I release the object will the kinetic friction disappear immediately proportional to exert force? And only static friction keep it deaccelerate?

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When I exert enough force that overcome the static friction. The object start moving and surface create kinetic friction on object if I exert harder overcome the maximum of friction it start accelerate. When I release the object will the kinetic friction disappear immediately proportional to exert force? And only static friction keep it deaccelerate?
When you release the object that was previously moving, kinetic friction does not know that you have released it.
Inertia keeps moving the object in the same previous direction.
Kinetic friction produces heat, which is energy that must come from the impulse of the object.
Its velocity and impulse decrease until they reach a value of zero and the object stops.

• topsquark
Clockclocle
When you release the object that was previously moving, kinetic friction does not know that you have released it.
Inertia keeps moving the object in the same previous direction.
Kinetic friction produces heat, which is energy that must come from the impulse of the object.
Its velocity and impulse decrease until they reach a value of zero and the object stops.
It mean when I stop pushing only the exert force disappear immediately but the friction remain still? I thought that kinetic friction is the reaction of the force which the object exert on the surface due to third law, why it don't disappear?

• PeroK
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It mean when I stop pushing only the exert force disappear immediately but the friction remain still? I thought that kinetic friction is the reaction of the force which the object exert on the surface due to third law, why it don't disappear?
You've already been told that friction has nothing to do with Newton's third law.

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The friction force depends only on the normal force and the coefficient of friction. Any tangential force has no effect on the magnitude of the friction.

Clockclocle
You've already been told that friction has nothing to do with Newton's third law.
it mean the reaction force the surface exert back to object isn't a friction?

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It mean when I stop pushing only the exert force disappear immediately but the friction remain still? I thought that kinetic friction is the reaction of the force which the object exert on the surface due to third law, why it don't disappear?
The amount of kinetic friction is only a percentage of that force that the object exerts on the surface while it slides over it (kinetic = movement).

As you know,
Kinetic friction = Dynamic coefficient of friction x Normal force

If the pushing force of your hand does not increase or decrease the normal force, because its pushing direction is perfectly perpendicular to it, it does not have any business in the value of the kinetic friction.

You push started the movement, changing the nature of friction from originally static (no movement) to kinetic (movement).
Once the object is sliding, you could remove your push and the thing would continue slidding forever: you gave it kinetic energy.

It does not slide forever in reality, because that kinetic energy is gradually consumed by friction (kinetic type).
The object then slowsdown until it stops (all the energy that you gave it is now higher temperature of the sliding surfaces in contact. Homework Helper
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it mean the reaction force the surface exert back to object isn't a friction?
Friction resists motion between two surfaces. Air resistance resists motion through the air. Water resists motion through it. These are resisting forces. They cannot initiate motion. Although, air and water currents can initiate motion.

They are not forces that exist because of Newton's third law. They exist because of the properties of materials. If you move through air you are colliding with air molecules and giving them kinetic energy, hence losing your own kinetic energy. That process of energy loss takes place regardless of whether you are being pushed through the air.

If you are moving through the air or water or sliding on a rough surface there is a resisting force regardless of any other forces acting on you.

These resistance forces are not caused by Newton's third law.

• davenn and Lnewqban
Clockclocle
The amount of kinetic friction is only a percentage of that force that the object exerts on the surface while it slides over it (kinetic = movement).

As you know,
Kinetic friction = Dynamic coefficient of friction x Normal force

If the pushing force of your hand does not increase or decrease the normal force, because its pushing direction is perfectly perpendicular to it, it does not have any business in the value of the kinetic friction.

You push started the movement, changing the nature of friction from originally static (no movement) to kinetic (movement).
Once the object is sliding, you could remove your push and the thing would continue slidding forever: you gave it kinetic energy.

It does not slide forever in reality, because that kinetic energy is gradually consumed by friction (kinetic type).
The object then slowsdown until it stops (all the energy that you gave it is now higher temperature of the sliding surfaces in contact.

Friction resists motion between two surfaces. Air resistance resists motion through the air. Water resists motion through it. These are resisting forces. They cannot initiate motion. Although, air and water currents can initiate motion.

They are not forces that exist because of Newton's third law. They exist because of the properties of materials. If you move through air you are colliding with air molecules and giving them kinetic energy, hence losing your own kinetic energy. That process of energy loss takes place regardless of whether you are being pushed through the air.

If you are moving through the air or water or sliding on a rough surface there is a resisting force regardless of any other forces acting on you.

These resistance forces are not caused by Newton's third law.
Im looking for an easier explanation but it seem like 3 Newton laws don't provide enough about the friction. According to Lnewqban, The friction only reduce to 0 but not make it move backward, why is it? Is there a change of kinetic friction reduce to static friction as the velocity become zero

Mentor
It mean when I stop pushing only the exert force disappear immediately but the friction remain still? I thought that kinetic friction is the reaction of the force which the object exert on the surface due to third law, why it don't disappear? https://en.wikipedia.org/wiki/Curling

• davenn and phinds
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Im looking for an easier explanation but it seem like 3 Newton laws don't provide enough about the friction. According to Lnewqban, The friction only reduce to 0 but not make it move backward, why is it?
Why would it move backward? Friction opposes motion in all directions. Physics describes the real world. You seem to have no understanding of the real world! The brakes on a car cannot send the car into reverse. Do you really expect that if you keep your foot on the brakes? That the car stops and then the brakes immediately drive the car into reverse? If you keep your foot on the brakes then the car cannot move in any direction. The brakes resist motion. But, they cannot accelerate the car.

• davenn
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I've never met anyone who thinks that if you put an object on a rough, flat surface that the force of friction spontaneously accelerates the object in a random direction. That's just a crazy idea. So crazy I wonder if you are trolling us?

Once an object has stopped on a rough, flat surface it's going nowhere under the force of friction.

Clockclocle
I've never met anyone who thinks that if you put an object on a rough, flat surface that the force of friction spontaneously accelerates the object in a random direction. That's just a crazy idea. So crazy I wonder if you are trolling us?

Once an object has stopped on a rough, flat surface it's going nowhere under the force of friction.
Because It make nonsense if friction remain still, it has to be reduced to make sure the object don't move backward. Friction proportional to exert force. When I realease the object it still move a little bit before stop then I guess the friction have to be reduced to its static form as the velocity of the object become zero

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Because It make nonsense if friction remain still, it has to be reduced to make sure the object don't move backward.
This is wrong.
Friction proportional to exert force.
This is wrong.
When I realease the object it still move a little bit before stop then I guess the friction have to be reduced to its static form as the velocity of the object become zero
This is incoherent. Static friction is greater than kinetic friction. That's wrong too.

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You've already been told that friction has nothing to do with Newton's third law.
You are, of course, correct that friction has little to do with Newton's third law. However, there is room for confusion.

In the case of static friction the frictional force will be whatever it has to be to prevent relative motion between the mating surfaces. In the usual case of a mobile object on a stationary surface, this means that the force of static friction will be equal and opposite to the sum of all other applied forces.

It is tempting to take this statement and say that static friction is a "reaction" to the other applied forces. It is then also tempting to think "equal and opposite" and "reaction". Well then, we must be talking about the third law!

Not so fast. That is actually what I like to call a "second law" action/reaction force pair. You have forces on an object whose momentum does not change (because it has negligible acceleration or negligible mass or both). The forces must sum to zero (##\sum F = ma##). So the one force must be equal and opposite to the sum of all the others.

As rules of thumb:

For static friction, you determine friction by summing the other forces on the object and negating the sum to determine both magnitude and direction of the frictional force. If the magnitude is within the bounds imposed by the coefficient of static friction, you are done. If the magnitude is too high, then slipping will be occurring. If not already, then after only an instant more.

For kinetic friction, you determine the magnitude based on the coefficient of kinetic friction. You determine direction by the opposite of the direction of relative motion. If the objects are at relative rest and about to begin slipping, you can determine the initial direction based on the opposite of the direction of the sum of the other forces.

• russ_watters
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Im looking for an easier explanation but it seem like 3 Newton laws don't provide enough about the friction. According to Lnewqban, The friction only reduce to 0 but not make it move backward, why is it? Is there a change of kinetic friction reduce to static friction as the velocity become zero
That is an interesting question.
At least in non-rigurous way, can you summarize the three laws and the two types of friction that you have learned for me?

Have you pushed a disabled car on a road?
Have you sanded wood with your bare hands?

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Because It make nonsense if friction remain still, it has to be reduced to make sure the object don't move backward.
When you release the pushing force on a moving object, then the friction force is equal to ##ma##. This means the object will decelerate, thus the velocity will get smaller. Once the velocity reaches zero, the object no longer moves, the kinetic friction doesn't exist anymore (by definition), the acceleration drops to zero, and the object velocity stays constant (at zero).

If the object is on an inclined plane (going against gravity), then gravity will play a role that will affect the acceleration, and when the object will stop (velocity = 0), the kinetic friction will be replaced by static friction. If static friction is not strong enough to hold the object still, then the motion will reverse, with static friction replaced by kinetic friction.

• PeroK, davenn and Lnewqban
Clockclocle
When you release the pushing force on a moving object, then the friction force is equal to ##ma##. This means the object will decelerate, thus the velocity will get smaller. Once the velocity reaches zero, the object no longer moves, the kinetic friction doesn't exist anymore (by definition), the acceleration drops to zero, and the object velocity stays constant (at zero).

If the object is on an inclined plane (going against gravity), then gravity will play a role that will affect the acceleration, and when the object will stop (velocity = 0), the kinetic friction will be replaced by static friction. If static friction is not strong enough to hold the object still, then the motion will reverse, with static friction replaced by kinetic friction.
Does the kinetic friction monotonically deacrease to 0 as the velocity become 0? Does it make velocity become zero and disappear immediately it mean the graph of kinetic friction respect to time t after I realease the object is continuous?

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Does the kinetic friction monotonically deacrease to 0 as the velocity become 0? Does it make velocity become zero and disappear immediately it mean the graph of kinetic friction respect to time t after I realease the object is continuous?
Kinetic friction retains its full value until velocity becomes zero. Then it abruptly switches to static friction. It behaves like a step function.

At least, that is the model presented in the first year physics classroom.

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• russ_watters, PeroK and jack action
Clockclocle
Kinetic friction retains its full value until velocity becomes zero. Then it abruptly switches to static friction. It behaves like a step function.

At least, that is the model presented in the first year physics classroom.
When I am riding a motocycle, suppose I slowly accelerate the car(push the pedal) the net force >0 for an interval of time, when it gets maximum the car move with constant velocity and net force become zero. Does it mean friction "come after" the exert force on the car?

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In this (motorcycle) case the drag forces increase with speed. "Frictional force" is an ill-defined physical quantity and one must be careful. A motor vehicle (or an object falling in air) will reach a terminal speed where the drag forces have increased with speed to match the impelling forces and acceleration is zero.
This is very different from the frictional forces at an interface which are modeled as a "stiction" and subsequent speed independent kinetic friction when motion commences.

We model all of these dissipative forces (drag, viscous drag, interface friction, etc) in various approximate ways. There is no fundamental "force of friction", although such forces are often useful to define for particular subjects and systems where there are things too complicated to consider in detail. Accept them as defined and use them as a tool as intended.

• russ_watters and jbriggs444