91Ó°ÊÓ

• The coefficient of static friction is ${\mathrm{μ}}_{\mathrm{s}}=1.2$.
• The coefficient of kinetic friction is ${\mathrm{μ}}_{\mathrm{k}}=0.90$.

The friction force always acts between two objects in contact. This force tries to restrict the motion of a particular object. Since the act of running starts with pushing on the ground, the greater the force exerted on the floor greater will be the acceleration. The value of maximum acceleration is constrained by static friction.

The maximum force that can be exerted is the static friction which is given by,

$$\begin{gathered} \mathrm{F}={\mathrm{μ}}_{\mathrm{s}}\mathrm{mg} \\ \mathrm{ma}={\mathrm{μ}}_{\mathrm{s}}\mathrm{mg} \\ \mathrm{a}={\mathrm{μ}}_{\mathrm{s}}\mathrm{g} \end{gathered}$$

Substitute 1.2 for ${\mathrm{μ}}_{\mathrm{s}}$in the above equation.

a= 1.2g

The maximum acceleration in horizontal direction is 1.2 g. So, the options (a), (b), and (c) are incorrect. Therefore, the correct answer is (d).