Question

A mass of 80 kg object is rest on a horizontal rough surface. Itscoefficient of static and kinetic friction are 0.21 and 0.18respectively.(a) Find the value of parallel force which just starts the object to move.(b) Find the value of parallel force which keeps the object in moving.

Answer 1

We will use the next diagram

Where Fr, is the frictional force, F is the applied force, W is the weight and N is the normal

For a)

We analyze the horizontal forces

$\begin{gathered} \sum ^{}_{}F_x=F_{}-Fr=0 \\ F=Fr \end{gathered}$

Then we analyze the vertical forces

$\begin{gathered} \sum ^{}_{}F_y=N-W=0 \\ N=W \end{gathered}$

then for the frictional force

$Fr=\mu_sN$

Then for the force we have

$F=\mu_sN=0.21(80)(9.8)=164.64N$

Then for b)

$F=\mu_kN=0.18(80)(9.8)=141.12N$

We