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As the Ping-Pong ball hits the paddle and gradually slides down, by analyzing the forces acted upon the ball, we can understand how vectors combine one another. There are 4 forces acted upon the ball in this scenario. There is a force due to gravity acts downward on the ball (Fg). Accordingly, there is an air resistance force acts upward on the ball as it slides down (Fair). Also, since the ball slides down on the surface of the paddle, there is a friction between the ball and the surface of the paddle acts upward on the ball (f). Plus, if we say that the player side is the horizontal-left side, as the ball strikes the paddle, the paddle applies a push force on the ball rightward (Fpush). However, since Ping-Pong ball’s mass is very small, thus in this case, force due to gravity (Fg) and air resistance (Fair) are relatively small comparing to friction (f) and paddle’s push force on the ball (Fpush). Thus, when doing force vectors analysis, it is fair to say that force due to gravity (Fg) and air resistance (Fair) are negligible. So, since friction (f) acts on the ball upward, and paddle’s push force acts on the ball rightward (Fpush), when combine these two force vectors, we can then get an oblique resultant force vector that makes an angle above the horizontal standard. The analysis of the vectors explains why Ping-Pong ball usually rebounds slant up after hitting by the paddle!