Yes. It is possible for the direction of acceleration to be opposite to the direction of velocity.
Acceleration is the rate of change of velocity. If velocity is increasing while moving in a straight line then the direction of acceleration is same as the direction of velocity.
It the velocity is decreasing whil travelling in a straight line then the direction of acceleration is opposite to the direction of velocity.
Example: When a stone is thrown vertically upwardsm its acceleration is downwards and velocity is upwards.
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