# Graph motion

in acceleration time graph when the acceleration is constant. Then at an instant it has negative acceleration. Does this mean that the body is decelerating or it is accelerating in the opposite direction. eg. http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/motgraph.html {last graph}

## Answers and Replies

When an object decelerates it usually means that the acceleration vector points in the opposite direction of the velocity vector. So when you apply a negative acceleration to an object it decelerates until its velocity reaches zero then it begins to accelerate in the direction opposite of the original motion.

how can u tell from the graph when the object comes to rest?

Redbelly98
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Phrases such as "comes to rest", "is at rest", and "starts from rest", all refer to the velocity. The answer lies in the velocity graph, because being at rest means that the velocity is equal to _____?

i was referring to the acceleration time graph.

Redbelly98
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It cannot be determined from the acceleration graph alone. You would need to know the velocity at some point in time, then it could be determined by taking the integral of the acceleration. (Have you taken a calculus course that covers integrals?)

my only problem is with the acceleration time graph. lets say a body is moving with constant velocity. THen in an instant it stops and accelerates in the opposite direction. Then wouldnt the graph be same as the graph mentioned above?

Redbelly98
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Then wouldnt the graph be same as the graph mentioned above?
No, the acceleration graph would be different:
my only problem is with the acceleration time graph. lets say a body is moving with constant velocity.
Okay. What is the acceleration while the body is moving with constant velocity?
THen in an instant it stops and accelerates in the opposite direction.
If it stops "in an instant", the acceleration would be -∞ for that instant. In actually, the body would stop in a very short time (not an really an instant), and the acceleration would have a very large magnitude and be negative.

Hope that helps.