A box has a weight of 50 N and it is slid along the floor at a constant speed by a force of 25 N as shown. Calculate:?

Question

(i) The friction force. 
[Answer: 19.15 N]

(ii) The coefficient of friction between the box and the floor.
[Answer: 0.56]

I do understand that for the first part you can get the Ff by finding x direction force (25cos40=19.15N ) but in second part im really confused. Im finding it hard to grasp the concept of friction. can somebody help

? · Accepted Answer

Friction can be difficult to grasp, especially static friction where we have two notions -- the MAXIMUM static friction (= µ * normal force) and the ACTUAL static friction (= and opposite to net force in direction of impending motion).

But you have kinetic friction here, and equilibrium ("constant velocity"), so
µ = friction force / normal force
and here specifically
µ = 19N / (50 - 16)N = 0.56

? · Answer

general equation :
(W-F*sin 40)*μ +m*a = F*cos 40 
constant speed means acceleration a = 0, therefore 
(W-F*sin 40)*μ +0 = F*cos 40

friction force FF = (W-F*sin 40)*μ = F*cos 40 = 25*0.766 = 19 N

friction coefficient μ = 19 / (50-25*0.643) = 0.560

A box has a weight of 50 N and it is slid along the floor at a constant speed by a force of 25 N as shown. Calculate:?

2 Answers