# Kinetic friction with no force on an object

We've learnt that friction is the opposition of motion and that friction appears the instant a force is applied on an object i.e when an object is at rest (with no force acting on it) then there is no frictional force. The moment a small amount of force is applied, friction becomes a factor. Therefore, friction is just the "equal and opposite" force between two bodies. Now, let an object be accelerated to a velocity 'v'. Then, let the acceleration cease. Ideally, the object will come to a stand still. However, if the acceleration is zero, doesn't that mean that there is no force => there will be no "equal and opposite" force i.e frictional force. And, only if there is an opposing force will there be retardation. Obviously, my reasoning is flawed, if not then an object that has been accelerated to a velocity will continue to move at a constant velocity. However, I don't get where my reasoning is flawed. Please do help...

• We've learnt that friction is the opposition of force and that friction appears the instant a force is applied on an objec Who taught you that? Can you provide evidence? This is all wrong/incorrect/vague. Perhaps you're confusing friction with inertia? Friction only occurs if objects with different velocities interact, for example a football travelling through air. – Walter Aug 3 '15 at 8:31
• I meant friction is the opposition of force. And, it's in my Physics given that static friction comes into effect the moment a force is applied on an object. Edited main post. – abhijeetviswa Aug 3 '15 at 14:38

Sorry, but the currently accepted answer is not precise enough.

In the situation you discussed, there is no applied force acting on the object, but there is still the normal force $N$, which prevents the object from "falling through" the surface due to gravity.

The most common model of kinetic friction is $F_k = \mu N$, where $\mu$ is the coefficient of friction between the interacting surfaces.

Since the object you discussed is in motion and is acted upon by a normal force, it is also acted upon by a friction force. From Newton's third law, the forced paired with this friction force is a friction force of equal magnitude but opposite direction that acts on the surface the object is sliding on. If our surface is as massive as the Earth, we can ignore any effects the paired friction force might have on it.

Remember that when you push the object with an applied force $F_A$, the equal-but-opposite force paired with $F_A$ is not the friction force, but rather the normal force exerted by the object on your hand. The friction in your scenario occurs between the object and the surface it is sliding on, not between the object and your hand.

• "Since the object you discussed is in motion and is acted upon by a normal force, it is also acted upon by a friction force." In which direction would this friction force be acted upon? And, you still have explained how an object with no force being acted upon it has kinetic friction acting on it. (Which should be the equal and opposite force between object and surface) – abhijeetviswa Aug 2 '15 at 3:02
• @Abhi2011 The person who first answered this question was correct in saying that friction arises when there is relative motion between two surfaces. But there is a second dependency: the normal force. Even if an object is sliding on a surface, if the surface is not exerting a normal force on the object, there will be no friction force. – Rations Aug 2 '15 at 3:24
• @Abhi2011 I think you are misunderstanding Newton's third law in this case. The relationship between the normal force on the object and the friction force on the object is not a third-law relationship. Rather, the actual third-law pair in this situation includes (1) the friction force exerted by the surface on the object and (2) the friction force exerted by the object on the surface. These two friction forces are equal in magnitude but opposite in direction. – Rations Aug 2 '15 at 3:28
• @Abhi2011 In other words, the fact that friction arises when two surfaces move against each other in the presence of a normal force is its own physical phenomenon; it really doesn't have to do with Newton's third law. – Rations Aug 2 '15 at 3:34
• Hello @Rations ,i sensed op was confused regarding when frictional forces actually come into existence so that was the only part i attempted to answer.Your answer is however, perfectly okay and more extensive. :) – Daipayan Mukherjee Aug 2 '15 at 4:50

Friction comes into play whenever there is relative motion between the surfaces in contact or a tendency of motion between the same.There need not necessarily be an externally applied force on either of the bodies,that is,there need not necessarily be a relative acceleration initially,merely relative motion or a tendency for the same. It is the frictional forces that produce this relative acceleration once the necessary conditions of relative motion have been established.

• So, friction is the opposition to relative motion or acceleration/force? – abhijeetviswa Aug 1 '15 at 17:01
• essentially it is opposition to relative motion or a tendency for relative motion. – Daipayan Mukherjee Aug 1 '15 at 17:02
• So, that'd be the flaw in my reasoning. Friction isn't the opposition to an applied force but rather to a tendency for relative motion or relative motion. So, for an object accelerated to 'v' and then no acceleration, friction is opposing the relative motion between the object and surface. That makes sense. Thanks :) – abhijeetviswa Aug 1 '15 at 17:21