Calculate the acceleration of the block and trolly system shown in the figure. The coefficient of kinetic friction between the trolly and the surface is $0.05.$ 
( $g=10~\text{m/s}^2,$ the mass of the string is negligible and no other friction exists)

See the figure given below, a mass of $4~\text{kg}$ rests on a horizontal plane. The plane is gradually inclined until at an angle $\theta=15^\circ$ with the horizontal, the mass just begins to slide. What is the coefficient of static friction between the block and the surface?

 
1. $0.27$
2. $0.53$
3. $0.23$
4. $0.25$