This is very trivial question and often you will find definition of rate of change of velocity is called acceleration in many physics literature but the fact is that you need to understand the proper meaning of rate of change of velocity what does happen when you accelerate your bike do you feel force yes it comes from Newton's second law that is force is the mass X acceleration the term acceleration is the rate of change of velocity.

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The rate of change of displacement of an object (displacement over elapsed time) is velocity. 
The rate of change of velocity is ACCELERATION. Like velocity, acceleration is a vector and has both magnitude and direction.

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