M1 help- Friction

**fishkeeper** · March 28th 2010, 12:46 PM

I cant seem to know where to start on this question:

A block is thrown down a slope inclined at 35 degrees to the horiztonal. It's initial velocity is $8ms^1$ and the coefficient of friction between the block and the slope is 0.8.
How long does it tale for the block to come to rest?

How do I try to start this please? I am thinking we may need to find the resistive force F and then use this to try and find a mass? And then use SUVAT to work out the time?

I am completely clueless it has to be said, and it would be amazing is anyone could possibly gie me some helpful pointers on where to go with this question?

many thanks, I will + rep you

**skeeter** · March 28th 2010, 01:02 PM

Originally Posted by fishkeeper

I cant seem to know where to start on this question:

A block is thrown down a slope inclined at 35 degrees to the horiztonal. It's initial velocity is and the coefficient of friction between the block and the slope is 0.8.
How long does it tale for the block to come to rest?

$F_{net} = ma$

$mg\sin(35) - (0.8)mg\cos(35) = ma$

$g\sin(35) - (0.8)g\cos(35) = a$

you also have $v_0 = 8$ m/s and $v_f = 0$ ... find $t$

**fishkeeper** · March 29th 2010, 03:13 AM

Thankyou Skeeter

I have followed your working and am glad to say I have understood

I completed this working:

I then got for a value of acceleration -0.80ms^2

As there was then $u=8, v=0, a= -0.8, t=?$

I then used v=u+at, rearranged to get $t= \frac {v-u} {a}$

$\frac {0-8} {-0.8} = 10s$

Is that the correct answer (Im just making sure)

Thanks so much!!!

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