# Friction and gravity

## Main Question or Discussion Point

The formula of friction force is

Ff = μN

can also be defined as

Ff = μ.m.g

One of my friend say that mathematically, that equation tell us that when there is a gravity force, Ff can't be zero assuming that there is no zero μ

Is that true? While on static condition I will say that even when there's a gravity force, the friction force is zero. But he said then if it is in static condition, then the formula would be

Fmax = μ.m.g

which mean when there is a gravity force, Fmax can't be zero.

Can physics really defined just by formula mathematically?

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tiny-tim
Homework Helper
The formula of friction force is

Ff = μN
No, that only applies to dynamic friction and when the bodies are about to become dynamic (ie Fstatic = Fmax).

In the static case, Ff < µN
… But he said then if it is in static condition, then the formula would be

Fmax = μ.m.g

which mean when there is a gravity force, Fmax can't be zero.
Yes, Fmax can't be zero, but Ff can be zero.

Yes, Fmax can't be zero, but Ff can be zero.
Is that true that the only condition Ff can be zero on gravity influence, is when the mass become completely static?

tiny-tim
I don't really understand what you're asking. 