# Acceleration for Position-time graphs

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## Answers and Replies

berkeman
Mentor
mathman
1. http://www.physicsclassroom.com/Class/1DKin/U1L3a16.gif
2. http://www.physicsclassroom.com/Class/1DKin/U1L3a17.GIF

In the first graph, I get that that the slope is negative and that the object is speeding up
And in the second graph, the slope is negative and the object is slowing down
But i'm unable to understand why the first graph has negative acceleration while the second one has positive acceleration?

Your question reversed the graphs?
In the first graph, the slope is getting steeper, speed up.
In the second graph, slope is getting less steep, slowing down.

Acceleration is the second derivative of the position versus time...
@berkeman Im sorry i dont understand how that relates to negative acceleration for the first and positive acceleration for the second

Your question reversed the graphs?
In the first graph, the slope is getting steeper, speed up.
In the second graph, slope is getting less steep, slowing down.

@mathman that is exactly what i mentioned above...

ZapperZ
Staff Emeritus
@berkeman Im sorry i dont understand how that relates to negative acceleration for the first and positive acceleration for the second

Do you understand the difference between the 2nd derivative of y=x2 and y=-x2?

Plot each of those two functions, and look at the nature of the curvature. One is curving UP, while the other is curving DOWN. So which one will give you a positive second derivative, and which one will give you a negative second derivative?

Zz.