Physics question...

Question

Physics question...

idku · Answer

Horizontal force $F=10~N$ is applied to a
stationary block with mass $M=2~kg$.

Find the acceleration of the block, if $\mu_s=0.710$, and $\mu_k=0.450$.

idku · Answer

It basically looks like this:

Friction      Pushing Force
 <----  ■ -------->

A 1D problem.

idku · Answer

Oh, zero is the correct answer !!

idku · Answer

It is not accelerating, because the static friction counteracts the acting force.

idku · Answer

I only figured this out when I posted the question, sadly. But, fortunately, did figure it out:)

michele_laino · Answer

at the beginning of motion, the friction force has the subsequent magnitude:
$$\huge   R = {\mu _S}mg$$

idku · Answer

Yes, and they didn't tell me acceleration at what moment or at what point, am I to find. So I presumed that I need an acceleration of the block at the very instant when the force is applied.

idku · Answer

And at that first instant as force is applied, a=0, because of the static friction.

I would also conclude, that for any positive friction coefficient, that this is the case.

idku · Answer

yes, 0 is right.

michele_laino · Answer

yes! the block will continue to be at the rest

idku · Answer

$f_s\ge$$\mu_s~m~g$

before motion begins, and when the motion begins, it is the very instant when this $f_s$ is exceeded. (Using the notion of limit, I can say that it is that very moment).

idku · Answer

i think it make sense, (and makes sense correctly) why motion begins with R=(μ_s)mg

michele_laino · Answer

that's right!

idku · Answer

tnx