A block of certain mass is placed on a flat surface. A force is applied to move it parallel to the surface. The frictional force fr developed is directly proportional to the :
Answer & explanation
Correct answer: option 2
Frictional force : $f_r = \mu N$
As N = mg
Thus, $f_r = \mu m g$
$\Rightarrow f_r \propto m$