Coefficient of friction for surface

[Mini_Mashi]

a) A block weighing 9.1N requires a force of 2.8N to push it along at constant velocity. What is the coefficient of friction for the surface?
b) A weight "W" is now place on the the block and 8N is needed to push them both at constant velocity. What is weight "W" on the block?

This is what I got:
Since it is constant velocity, there is no acceleration, therefore Fnet is 0, therefore Ff is 2.8N.
Normal force is (9.1)(9.8) =89.18N
Formula: Ff=uN

2.8=u(89.18)
2.8/89.18=u
Therefore u= 0.031
, but that's not the right answer... Why? what did I do wrong?

I take it I need to know a before I can do b?

 

[jamesrc]

Why did you multiply the weight of 9.1 N by 9.8 to get the normal force?

 

[Mini_Mashi]

Why did you multiply the weight of 9.1 N by 9.8 to get the normal force?

Hm? Don't you need to?
Fg=mg so Fg=(9.1)(9.8), and normal force is going the other direction and Fnet=0, so normal force is also 89.18.

Isn't that right?

 

[jamesrc]

The weight would equal the mass times g, but you weren't given the mass, you were given the weight which is equal to the normal force. Look at your units: N is a unit of force, if you were given mass in kg and multiplied that by g which has units [m/s^2], you would end up with Newtons (again, a force). But what is 1N * 1m/s^2?

 

[Mini_Mashi]

The weight would equal the mass times g, but you weren't given the mass, you were given the weight which is equal to the normal force. Look at your units: N is a unit of force, if you were given mass in kg and multiplied that by g which has units [m/s^2], you would end up with Newtons (again, a force). But what is 1N * 1m/s^2?

ohhh. I see what I did wrong, thanks! You're great help!
I can't believe my mistake!