A block of certain mass is placed on a rough floor. The coefficients of static and kinetic friction between the block and the floor are 0.4 and 0.25 respectively. A constant horizontal force $\mathrm{F}=20 \mathrm{~N}$ acts on it so that the velocity of the block varies with time according to the following graph. The mass of the block is nearly (Take $\mathrm{g} \simeq 10 \mathrm{~ms}^{-2}$ )

A body of mass 0.25 kg travels along a straight line from $x=0$ to $x=2 \mathrm{~m}$ with a speed $v=k x^{3 / 2}$ where $k$ $=2 \mathrm{SI}$ units. The work done by the net during this displacement is
During an elastic collision between two bodies, which of the following statements are correct?
I. The initial kinetic energy is equal to the final kinetic energy of the system.
II. The linear momentum is conserved.
III. The kinetic energy during $\Delta \mathrm{t}$ (the collision time) is not conserved.
Three particles of mass $1 \mathrm{~kg}, 2 \mathrm{~kg}$ and 3 kg are placed at the vertices A, B and C respectively of an equilateral triangle ABC of side 1 m . The centre of mass of the system from vertex A (located at origin) is