The sum of moments of all the particles in a system about its center of mass is always______

A particle of mass $\text{'}m\text{'}$, moving with a velocity $\text{'}v\text{'}$ makes an elastic collision in one dimension with a stationary particle of mass $\text{'}m\text{'}$. During the collision, they remain in contact with each other for an extremely small time $\text{'}T\text{'}$: Their force of contact, with time is shown in the figure. Then $F_0=$

A ball of mass $3 \mathrm{kg}$, moving with a speed of $100 \mathrm{m} {\mathrm{s}}^{-1}$, strikes a wall at an angle $60°$ (as shown in figure). The ball rebounds at the same speed and remains in contact with the wall for $0.2 \mathrm{s}$; the force exerted by the ball on the wall is: