Because there is no friction between objects and between object and the ground.
The total momentum in the horizontal direction must be conserved.
Try with different initial velocity and water how the motion changed!
The total energy of the system is always conserved,too!

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Enjoy the fun of Physics! You can access this simulation online from
A ball climb up another object on a frictionless surface
Designed by Fu-Kwun Hwang (http://www.phy.ntnu.edu.tw/ntnujava/)
Deptartment of Physics, National Taiwan Normal University, Taiwan