The gradient of a velocity/time graph..

[mahela007]

Say an object A changes it's velocity from -3 ms-1 to 0 ms-1. Commons sense suggests that this is a decelleration because the speed of the object is decreasing. However if this motion is drawn on a velocity time graph then the gradient will be positive. A positive gradient means acceleration... (correct me if i'm wrong anywhere above)..

So we knows it's a decelleration but the graph says it's an acceleration.. how can you account for this discrepancy?

[insane_alien]

technically there is no such thing as deceleration, there is only acceleration.

 

accelreation is a vector which means it has magnitude and direction. in this case the acceleration is in the positive direction and 3m/s /t in magnitude.

 

you get a discrepancy by mixing scalars and vectors without thinking about it. you expect the acceleration to be negative because the velocity is becoming less but you completely ignore the direction of the velocity. to slow down when it is travelling backwards it must accelerate forwards.

[Sisyphus]

As insane alien said, there's actually no such thing as decceleration in physics. Acceleration is just a change in velocity. And velocity is a vector - it has magnitude and direction. What in your problem is called a velocity of -3 is really just a velocity of 3 in the direction we've arbitrarily decided to call "negative." This is the same as saying it has a velocity of -3 in the opposite direction, that we've arbitrarily decided to call "positive."

[proton]

As insane alien said, there's actually no such thing as decceleration in physics. Acceleration is just a change in velocity.

The term decelleration is a well defined term in physics. It refers to an acceleration which results in a decrease in speed. You can find it defined in Fundamentals of Physics - Third Edition by Halliday and Resnick, page 18 - footnote

The word deceleration is often used to mean a decrease in speed.

[insane_alien]

1/ your reference says 'often' 9/10 this means 'the common usage of the word is...'

 

2/ this in no way changes the answer.

[proton]

1/ your reference says 'often' 9/10 this means 'the common usage of the word is...'

What's your point? I was merely pointing out that it is a term which is well defined in physics and as such it's incorrect to say technically there is no such thing as deceleration. It's merely a term which isn't used much. I myself have never found cause to use it in fact. Edited June 25, 2009 by proton