# Kinetic friction and applied force

I think this question is not important and rarely noticed, but I'm still curious XD
kinetic coefficient of friction is used when the applied force of static friction coefficient is less than than the force that we give, or less than or equal to the force that we give?

I think the case would be like the former more. When they are equal the net force is still 0 Newton. (right?)
This situation I think can be analogous to that in photoelectric effect. When the given energy is equal to the work function, there's no electron emitted.
Above is just my ideas.

• diazdaiz
robphy
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Gold Member
The kinetic coefficient of friction is used when there is relative motion between the surfaces of contact.
If there is no relative motion between the surfaces of contact, then you don't use kinetic friction.

• Chestermiller and diazdaiz
CWatters
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+1

I think this question is not important and rarely noticed

It is important and noticed.

kinetic coefficient of friction is used when the applied force of static friction coefficient is less than than the force that we give, or less than or equal to the force that we give?

IF static friction is less than the applied force AND that causes the object to slide THEN use the kinetic coefficient of friction.

Normally if the static friction is equal to the applied force the object won't move (it has to be greater for the object to accelerate).

Note that it is usually the net force on an object that determines if it moves. That is not always the same as the applied force.

kinetic coefficient of friction is used when the applied force of static friction coefficient is less than than the force that we give, or less than or equal to the force that we give?
even use when net force = friction and v nonzero

robphy
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Gold Member
even use when net force = friction and v nonzero

It's important to keep in mind that there is kinetic (sliding) friction
when there is a nonzero RELATIVE-velocity (i.e. SLIDING) between the surfaces.

A box sits on rough top of an moving accelerating truck.
Even though the box has nonzero velocity,
if the box is not sliding on the surface of the truck, the friction force on the box in that situation is static friction (not kinetic friction).

A box sits on rough top of an moving accelerating truck.
Even though the box has nonzero velocity,
if the box is not sliding on the surface of the truck, the friction force on the box in that situation is static friction (not kinetic friction).
thanks