Question

A box rests on a rough board 10 meters long when one end of the board is slowly raised to a height of 6 meters above the end, the box begins to slide the coefficient of static friction is :
A) 0.8
B) 0.25
C) 0.4
D) 0.75

Answer 1

Hint: Friction is the force which bears or resists the relative motion of solid surfaces sliding against each other. There are different types of frictions and that is dry friction, fluid friction, lubricated friction, skin friction and internal friction. Friction is measured in terms of force so the unit of friction is newton.

Complete answer:
Static friction can be defined as the tendency of the relative motion when the object is at rest. If the normal force exists between the two surfaces and we slide the object, then the force of static friction acts even before sliding.
Coefficient of friction is defined as the ratio of the force of friction between two bodies and the force pressing them together.
When a box rests on a rough board 10 meters long when one end of the board is slowly raised to a height of 6 meters above the end, the box begins to slide which you can observe in the diagram below.

Therefore
 $\begin{align}
  & \tan \theta =\dfrac{6}{8} \\
 & \tan \theta =0.75=\mu \\
 & \mu =0.75 \\
\end{align}$
So the correct option is D.

Note: The various examples of static frictional force is supposed to be a car rolling down a steep slope. Then when the brakes are applied, the car moves a bit further down the steep road but then eventually comes to a halt. At this position, the static frictional force is the force which is stopping the slipping of the wheel of the car from occurring.