What is the role of friction in Newton's Laws?

[KiNGGeexD]

Hi I am stuck with a question which I have attached a photo of, it is one involving friction! I have done a few of these questions already and find it is when I am defining forces etc it's where I go wrong, not the easy computational part:(

I also have attached my attempt at the solution and have drawn my free body diagram as clear as possible:)Any help would be great:) thanks

 

[voko]

I see no mistake in your attempt so far.

 

[KiNGGeexD]

Excellent I shall continue onwards and reply shortly:)

 

[KiNGGeexD]

Ok I went on and done this to find the acceleration but the model answer is 3.71 m/s^2

 

[voko]

In your computation you assumed that static friction will not be strong enough to prevent motion. Part (a) of the problem required you to determine whether this was the case.

 

[KiNGGeexD]

It is not strong enough

 

[KiNGGeexD]

It is the two fractions which are confusing me, kinetic friction is the friction between two moving bodies and static is between two non moving bodies would there be a case where both would have to come into a computation?

 

[voko]

Can you demonstrate that?

 

[voko]

I did not see #7 when I wrote #8. Please explain how you determined that static friction was not strong enough to prevent motion. That will help clarify what you confusion really is.

 

[KiNGGeexD]

So for the block to move mg sinθ must be greater than static friction?

 

[KiNGGeexD]

Sorry F cosθ

 

[voko]

Yes, the horizontal component of force must be greater than the maximal static friction. The maximal static friction is given by $\mu_s N$.

 

[KiNGGeexD]

 

[voko]

I agree with your result, but it is not correct to write $F \cos \theta - \text{fr} = 0$. You should write $F \cos \theta - \text{fr} > 0$.

 

[CWatters]

Check what value you used for μ.

I put 0.2 into your earlier equation and got 3.7 as the answer.

 

[KiNGGeexD]

Oh! Haha I think I've got it now lol