Dear Student
It is because of newton's 3rd law
Every action has equal and opposite reaction.
Ball exert a force on wall in turn wall also exert same amount of force on ball.
 

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AKASH GOYAL
AskiitiansExpert-IIT Delhi
 
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A ball has kinetic energy when thrown towards wall
now  after collision with the wall the next motion depends on how the energy is transferred and where, because energy is always conserved and can't just disappear
the wall of course doesnt move and no sufficient heat is produced  to compensate loss of kinetic energy
hence ball has to continue moving but by newtonian mechanics it is clear that it will move backwards coz wall exerts equal n opp. force on it
so ball bounces backwards and if it is perfectly rigid it will do so with the initial speed