Acceleration refers to– increasing in the rate or speed of something. In other words, becoming or going faster. For example- a thing falling down towards the earth falls 10 meters per second faster in every second. If it starts from rest its speed is 10 meters per second at the end of the first second, 20 meters at the end of the second, etc. Its acceleration is 10 meters per second, per second. So also acceleration of a bullet, car – or of a process, such as- reproduction of bacteria, spread of chemical change through an explosive, etc.
Reference: An Elementary Scientific and Technical Dictionary
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