Solving a Friction Force and Coefficient of Friction Problem

[nitegrl]

Hi I'm new here, I hope someone can help me. I think I got the first half of the question, but I got stuck one the second part. Well here it goes,

A force of 40N accelerates a 5.0kg block at 6.0m^2 along a horizontal surface.
a.) How large is the friction force?
b.) What is the coefficient of friction?

a.) Fnet= ma
Fp-Ff= ma
(40)-Ff= (5)(6)
40-Ff= 30
40-30= Ff
Ff= 10N

I know that you have to use Ff= (mu)Fn, but how do you get Fn?

And is there a way to show symbols like mu?

 

[Leong]

Since the block moves in the horizontal direction only, the sum of the vertical forces equals zero. that is how you get fn.
type to get the symbol.
$$\mu$$

 

[Tom McCurdy]

$$fn=mg=9.8*5*\mu$$
$$10=49\mu$$
$$\mu=.2$$

does this seem right? its been awhile