question_answer

    A block moving on a surface with velocity 20 m/s comes to rest because of surface friction over a distance of 40 m. Taking\[(g=10m/{{s}^{2}})\], the coefficient of dynamic friction is

A)  0.5                                        

B)  0.3

C)  0.2                                        

D)  0.1

Correct Answer: A

Solution :

                Key Idea: Final velocity is zero. From equation of motion \[{{v}^{2}}={{u}^{2}}-2as\] where v is final velocity, u the initial velocity, a the acceleration and 5 the displacement. Given,     \[v=0,\text{ }u=20\text{ }m/s,\text{ }s=40\text{ }m\] \[\therefore \]  \[0={{(20)}^{2}}-2\times 40\times a\] \[\Rightarrow \]               \[a=5\text{ }m/{{s}^{2}}\] Kinetic (or dynamic) friction occurs when two objects are moving relative to each other and rub together. It is -given by                 \[{{\mu }_{x}}=\frac{a}{g}=\frac{5}{10}=0.5\]