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1.  the acceleration due to gravity g = G M / (Re+h)²
    Gravity is less at an  altitude h from the surface of Earth.  Hence, the ball bounces higher.    Suppose the ball rebounds with a velocity v. 
        S = distance it rises to = v² / 2 g.   

    As, g is less on mountains, S is more.

2.  Also, air pressure is less.  The drag resistance due to air is less.  So the ball bounces higher.

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Tennis ball bounce higher on hills than in plains because of low gravity . Means that , the force of the Earth is less at these places . 
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