# Can an object be said to: accelerate from 50 km/h to 0 km/h? Or must one say that the object has decelerated?

Asked by: John Galt### Answer

**Acceleration is the change of velocity per unit time.**

This change can be either positive (implying an increase in velocity) or it can be negative (implying a decrease in velocity). The term 'accelerate'can can be applied to both 'speeding up' and 'slowing down' situations. So the answer to your question is yes one can say that an object accelerated from 50 km/h to 0km/h. In fact to use the term accelerate is more scientifically correct than decelerate when accompanied by the words 'negative' or 'positive'.

Answered by: Laila A.

The quick answer is 'accelerate it from 50 km/h to 0 km/h'. Let's see why...

When you have a certain magnetized piece of iron (say, your screwdriver) and you want someone to make it 'not magnetized', how would you ask? I think you would ask 'Hey, can you please 'demagnetize' this for me?'

Ok, now you have a certain object with constant speed of 50 km/h. It is NOT accelerated. How can you decelerate it if it's not accelerated? You certainly would say that it needs to be accelerated, with the direction opposed to the object movement, down to 0 km/h.

Let me tell you that when an object is accelerated in a certain direction the object itself does NOT need to be moving in that direction: when you are braking you car you are just accelerating it in the opposite direction of the car trajectory.

Answered by: Artur Adib, Undergraduate Physics Student, UFC-Brazil

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