[填空题]
While practising, a table tennis player hits a ball of mass $2.8\,g$ that collides with the floor and then bounces back from the wall. The speed of the ball just before reaching the floor is $8.0\;m\;s^{-1}$. The ball leaves the floor with an angle of $65^{\circ}$ to the floor as shown.
1.Due to the collision with the floor, the ball losses 30% of its initial kinetic energy. The leaving speed of the ball from the floor is around $m\;s^{-1}$.
2.The ball strikes the wall just as it reaches the highest point of its motion. Determine the horizontal distance between the bounce point and the wall. $m$
3.The ball collides elastically with the wall, with the time of contact during the collision is $0.004\;s$. Calculate the average horizontal force exerted by the wall on the ball during the collision. $N$
