Question

A $20kg$ metal block is place on a horizontal surface. The block just begins to slide when horizontal force of $100N$ is applied to it.
Calculate the coefficient of static friction.

• A. 74.8 N
• B. 104 N
• C. $\sqrt{84}N$
• D. 100 N

Answer 1

The correct option is A 74.8 N

We have

$f={\mu }_{s}mg100={\mu }_s\left(20\right)\left(9.8\right){\mu }_s=\frac{100}{20\left(9.8\right)}=0.51$

Now using Newton's second law

$\sum \overrightarrow{f}=m\overrightarrow{a}f-\mu kN=0$ because acceleration has to be $0$

$f-\mu kmg=0f-\left(0.4\right)\left(20\right)\left(9.8\right)=0f=78.4N$