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I know that if an object is on an incline, there is friction as a reaction to the component of the object's mass that is parallel to the surface. I know that if an object is on a flat surface and an external, horizontal force is applied, there is a friction force as a reaction.

But what about an object sitting on a flat surface with no external force? Does the friction force not "exist" or not apply? If I were to draw a FBD, would it just be the gravitational force and normal force reaction?

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  • $\begingroup$ Make the free body diagram, to check if there is supposed to be an unbalanced horizontal force which should be balanced by frictional force. If there is none, then there isn't any frictional force. $\endgroup$
    – Eagle
    Commented Sep 28, 2019 at 19:30

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I know that if an object is on an incline, there is friction as a reaction to the component of the object's mass that is parallel to the surface.

It is more accurate to say that there is static friction acting up the plane that is equal and opposite to the component of the gravitational force acting on the object down the plane. Static friction prevents motion of the object down the plane. But if the component of the gravitational force acting down the plane exceeds the maximum possible static friction force, the object will slide down the plane. Then the friction force acting up the plane becomes the kinetic friction force. If you do a free body diagram as @Eagle suggested, you will see this.

I know that if an object is on a flat surface and an external, horizontal force is applied, there is a friction force as a reaction.

That is correct.

But what about an object sitting on a flat surface with no external force? Does the friction force not "exist" or not apply? If I were to draw a FBD, would it just be the gravitational force and normal force reaction?

A friction force only exists in opposition to an external applied force. It is zero if the applied force is zero. As an applied external force increases, the static friction force increases by the same amount so that the net force is zero and there is no relative motion between the object and the surface. This continues until the maximum static friction force is reached, which is for a flat surface $f_{max}=μmg$ where $μ$ is the coefficient of static friction. Then the object slips and kinetic friction takes over.

Hope this helps.

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  • $\begingroup$ That makes sense, thanks! As reflected in what Sam answered, if there is absolutely no horizontal force, then the friction force is 0, and the second the horizontal force is non-zero, the friction force cancels it (until the maximum static friction force is reached, and then there is kinetic friction). Makes sense now! $\endgroup$
    – Joshua O'Reilly
    Commented Sep 28, 2019 at 21:05
  • $\begingroup$ @JoshuaO'Reilly You got it down pat. I've up voted Sam's answer too. $\endgroup$
    – Bob D
    Commented Sep 28, 2019 at 21:34
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Friction is a self-adjusting force that opposes relative contact motion between two surfaces. If a surface is not perfectly smooth, friction will always be there. If a block is on a flat surface, friction adjusts itself to zero as there is no tendency for the block to rub against the flat surface. If you were to apply even the slightest force in the horizontal direction, the frictional force would try to counteract that.

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  • $\begingroup$ Sam- nice compact answer. $\endgroup$
    – Bob D
    Commented Sep 28, 2019 at 21:35
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Consider a body on a surface. What we call the friction between the body and the surface is actually the averaged effect of interactions between countless atoms where the body and surface meet. When the body is stationary on the surface, all of those individual interactions between atoms cancel each other out- they are in equilibrium, and the net force is zero. So, overall there is no net frictional force acting on the body- if that wasn't the case, friction would cause the body to move.

When a small external force is applied to the body it will shift fractionally and all the millions of tiny forces will change somewhat, some increasing and some relaxing until equilibrium is reached again. If the force is gradually increased the body will continue to move fractionally until the point at which equilibrium can no longer be maintained- beyond that point further increases in the external force will accelerate the body across the surface.

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    $\begingroup$ "When the two surfaces are stationary with respect to each other, all of those individual interactions between atoms cancel each other out". You might want to consider revising this. This makes it sound like there is no friction if there is no relative motions between the surfaces. That would not be correct since you can have static friction that opposes an external force keeping the two surfaces stationary with respect to each other. The statement would be correct if you were referring to kinetic friction. $\endgroup$
    – Bob D
    Commented Sep 28, 2019 at 21:40
  • $\begingroup$ That's not true. When the external force is applied there is a minute movement between the two surfaces which adjusts the equilibrium between all the countless interactions. As the external force is increased the tiny movement increases until equilibrium can no longer be maintained, $\endgroup$
    – Professor Sushing
    Commented Sep 29, 2019 at 10:58
  • $\begingroup$ That “movement” as you call it should not be confused with the macroscopic relative motion between surfaces that is normally associated with friction. That’s my only point. $\endgroup$
    – Bob D
    Commented Sep 29, 2019 at 12:07
  • $\begingroup$ Agreed. I will try to find time to clarify my answer. $\endgroup$
    – Professor Sushing
    Commented Sep 29, 2019 at 12:09
  • $\begingroup$ Just a suggestion, but I would describe it as the microscopic movement associated with the interlocking of the adjacent surfaces thereby resisting macroscopic relative motion between them, or something along those lines $\endgroup$
    – Bob D
    Commented Sep 29, 2019 at 12:17
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If maximum static friction force would be expressed in vector form it would be: $$ \large{\vec{F}_{max} = -\mu_{s}\cdot\|\vec{F}_n\| \cdot \hat{\textbf{u}}_{\perp F_n}} $$ here,

  • $\mu_{s}$ - static friction coefficient
  • $\vec{F}_n$ - normal force
  • $\hat{\textbf{u}}_{\perp F_n}$ - unit vector of applied external force perpendicular to normal force vector

So, there is no static friction force if :

  • surfaces are friction-less
  • no normal force is exerted on body (for example, weightlessness mode in free-falling bodies)
  • no external force applied perpendicular to normal force

As a side-note,- if static friction force would exist without external force applied,- then body which have been put on flat surface, would have started to move itself with acceleration (second Newton law), without any apparent reason. Thus it would break causality.

However, surface features which causes friction force,- called asperities-, exists always no matter friction force is induced or not. Asperities: enter image description here

As a bonus points. Typical earthquake is caused by the same friction mechanism when static friction force is not able to hold-on pair of drifting tectonic plates glued together anymore.

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