When is this statement true?

Question

I am assuming Kinetic friction or force is equal to product of the product of the friction coefficient and mg. ( Ff = coefficient x mg)

and that this force is somehow equal to ma thus giving us
a = coefficient x g

Answer 1

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Your assumption that kinetic friction is equal to product of coefficient of kinetic friction and mg is true if the rough surface on which the body of mass m is moving is horizontal.

Kinetic friction force = Ff =coefficient of kinetic friction x mg

where m is mass of moving body and g is acceleration due to gravity.

This force of kinetic friction is equal to ma thus giving us
a = coefficient x g ONLY WHEN force of kinetic friction is the NET FORCE on the body

For example when you give a push to box on rough horizontal surface and the box is sliding after you have stopped pushing it, then the force of kinetic friction is the net force and at that time retardation of the box is ' a' = coefficient of kinetic friction x g

If there is some external applied force F is acting on the moving body , then

F - coefficient of kinetic friction x mg = ma

F - Ff = ma

and a = [ F - Ff ] / m

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Answer 2

It's only true in a few cases, not in general.

The kinetic friction force is given by the friction coefficient times the normal force pressing the contacting surfaces together. That normal force is *not* in general mg -- it is only when you have a mass m supported by a horizontal surface.

The only time the friction force is equal to ma is if the friction force is the total net force acting in the direction of the acceleration. That's not always the case.

Be careful ...

Answer 3

Yes

Answer 4

Gibberish.

Answer 5

If you are considering an object moving on a horizontal surface then your equations are valid.

Answer 6

Could I have some more scotch, before answering that !?

Answer 7

i think this is correct

Answer 8

what is the question?