The force on the ball is unbalanced, the floor exerts a force on the ball which causes the ball to accelerate away from the floor. However, this doesn't explain why the ball changes its shape. If you hold the ball in your hands and compress it, it will also change shape, but now there is a balanced force acting on it.
So, why does the ball change shape? If you divide up the ball into small mass elements then the ball will change shape if these mass elements don't move toward or away from each other. If they start out moving woth zero relative velocity, then the ball will change shape if the mass elements get displaced relative to each other, which means that the relative velocities must have been nonzero, which in turn implies that the accelerations on the different mass elements must have bee unequal, and that then means that the forces on the mass elements were unequal.
The reason why the ball changes shape is then due to non-uniform forces acting on the ball. When the ball hits the ground, at the moment of impact, the floor exerts a force on the ball, but this force only acts on the outer part of the ball that is in direct contact with the ball. This part of the ball decelerates, while the opposite part of the ball continues to move toward the floor for a fraction of a second until a pressure wave propagating through the ball hits it.
When a basketball changes shape as it bounces off the floor is it because of balanced force acting on it?
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