What Is the Correct Coefficient of Friction Calculation?

[anglum]

A block weighing 8.1 N requires a force of
2.8 N to push it along at constant velocity.
What is the coeficient of friction for the
surface?

002 (part 2 of 2) 10 points
A weight W is now placed on the block and
4.8 N is needed to push them both at constant
velocity.
What is the weight W of the block? Answer
in units of N.



For part 1 i used the equation 2.1N = Cf * 8.1N
and solved for Cf or coefficient of friction

that was incorrect tho

 

[Doc Al]

For part 1 i used the equation 2.1N = Cf * 8.1N
and solved for Cf or coefficient of friction

Shouldn't that be 2.8 N? (Assuming the force is applied horizontally.)

 

[anglum]

yeah i mistyped that it should be

2.8N = Friction * 8.1N

Friction = .34567 ?

is that correct? if i have that then i can solve the 2nd one

by taking
4.8= .345679 *(8.1N+W) and solve for W?

 

[anglum]

and in solving for W there i get 5.7857 N

 

[Doc Al]

Looks good to me. (You may want to round off your answer to a reasonable number of significant figures--especially if you're using an online system.)