Finding the coefficient of friction

[xSnoopy]

I need help with my homework.

http://img36.imageshack.us/img36/8717/friction.png

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I don't how to figure this out because you only get velocity :/

 

[rock.freak667]

Well on the horizontal block, what are the forces acting?

On the vertical block, what are the forces acting?

Find the resultant force on each block. What do you get?

 

[xSnoopy]

Well its not at equilibrium so i can't just sum the forces to get zero ...?

 

[rock.freak667]

Well its not at equilibrium so i can't just sum the forces to get zero ...?

That is why you put the resultant force as 'ma'

 

[xSnoopy]

so Force normal = 2.94N
and force moving horizontal would be (0.3)(a)

then
μ(2.94) + (0.3)(a) = (0.3)(a) ? ... crap I'm not getting something here :/

 

[rock.freak667]

so Force normal = 2.94N
and force moving horizontal would be (0.3)(a)

then
μ(2.94) + (0.3)(a) = (0.3)(a) ? ... crap I'm not getting something here :/

Consider the horizontal and vertical blocks separately.

On the horizontal block you have a tension T

so T-μ(0.3*9.81) = 0.3a

now do the same for the vertical block.

You will now have two equations in T and a. Eliminate 'T' from the two, leaving one equation in 'a'.

 

[xSnoopy]

Oh like net force?

The vertical would then be..

T - 0.3a = 0.3a ?

 

[rock.freak667]

Oh like net force?

The vertical would then be..

T - 0.3a = 0.3a ?

Vertically, you'd get mg-T, since the block moves down.

 

[xSnoopy]

So you would get

T-μ(0.3*9.81) = 0.3a
(0.3*9.81) - T = 0.3a

and substitute ?..

T-μ(0.3*9.81) = (0.3*9.81) - T

that doesn't cancel out the T's ?

 

[rock.freak667]

So you would get

T-μ(0.3*9.81) = 0.3a
(0.3*9.81) - T = 0.3a

and substitute ?..

T-μ(0.3*9.81) = (0.3*9.81) - T

that doesn't cancel out the T's ?

No. If you add the two equations together, what do you get?

 

[xSnoopy]

You get

μ(2.94) + (2.94) = 0.6a ?

 

[rock.freak667]

right, so you can find 'a' in terms of 'μ'. You also know that 'a' is constant. Now use a kinematic equation involving distance, time and acceleration.

You should now be able to get the value of μ.