A rubber ball of mass m and radius R is filled with air. The excess pressure inside the ball is $Δ P .$ However, mass of the air in the ball is negligible. The ball surface is flexible but non-stretchable. When you push its surface at a point with your finger, the ball’s surface in contact with your finger gets flattened as a circle of area A. In this position, the ball exerts a force $Δ P A$ on your finger. We will assume that when A is small, the excess pressure remains unchanged. Such a ball strikes a vertical wall when moving horizontally and rebounds with its speed unchanged. The ball remained in contact with the wall for a time interval $Δ t .$ Assume the deformation in the ball to remain small.
Contact area between the finger and the surface of the ball is A. Force between them is $Δ P A .$

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