# Does a decreasing acceleration mean a body is decelerating ???? ASAP

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Thread starter 6 years ago
#1
I was just looking at a velocity time graph and it shows the acceleration decreasing i.e going from a more steep gradient to a less steep gradient, I know the acceleration is decreasing but does this mean it is decelerating as I thought if a body was decelerating it would have a negative gradient on a velocity-time graph not a positive gradient ? Thanks help is much appreciated
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6 years ago
#2
(Original post by emmalav)
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First off, I'm sure you know that acceleration is the rate of change of velocity. It's a measure of how the velocity is changing.

If the acceleration is greater than zero, the velocity will always be increasing. In your case, acceleration is decreasing but is still greater than zero. Hence, the velocity is still increasing and this is shown by a positive gradient on a velocity-time graph.

Deceleration only occurs if acceleration is less than zero.
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6 years ago
#3
(Original post by emmalav)
I was just looking at a velocity time graph and it shows the acceleration decreasing i.e going from a more steep gradient to a less steep gradient, I know the acceleration is decreasing but does this mean it is decelerating as I thought if a body was decelerating it would have a negative gradient on a velocity-time graph not a positive gradient ? Thanks help is much appreciated
Acceleration is rate of change of velocity. Deceleration means that the objects velocity is reducing:

If the acceleration has changed and the gradient is still positive, it means that the rate of change of velocity has changed but the object is still accelerating nonetheless. i.e. its velocity is not decreasing but still increasing albeit at a lower rate.

Acceleration is the result of a force acting in the object. Newton's laws of motion:
An object will continue at rest or in its line of motion unless acted on by an external force.

Therefore if an object is accelerating, there must be an external force acting on it to produce the acceleration.
If the acceleration changes, then the force acting to produce the acceleration has changed. This is what is shown happening when the gradient of the velocity-time graph reduces, but as long as the gradient is still +ve, the object must still be accelerating.

If the gradient is zero (i.e. the V/time graph shows a horizontal line) then there is no force acting on the object and it will therefore move in a straight line at a constant velocity as shown by the V/t graph.

To decelerate the object, a force in the opposite direction to its line of travel must be present and this will produce a negative gradient as you rightly pointed out. At which time the actual velocity will start reducing. When the velocity gets to zero but the force that produced the deceleration is still present, the object will start accelerating in the opposite direction.
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