A cube bar slides thrice the time with friction than that without friction when it slides on an inclined plane of inclination $$45^{\circ}$$. The coefficient of friction between the block and the surface is

A man standing in a truck moving with constant velocity throws a ball vertically into air, ball falls

A stone of mass $$2 \mathrm{~kg}$$ is hung from the ceiling of the room using two strings. If the strings make an angle $$60^{\circ}$$ and $$30^{\circ}$$ respectively with the horizontal surface of the roof then the tension on the longer string is : $$g=10 \mathrm{~ms}^{-2}$$

If the resultant of all external forces acting on a system of particles is zero, then from an inertial frame one can surely say that