On a displacement/ time graph, what does a curved line represent?
It indicates that the object has an acceleration. If the curvature is concave-up, like a smile, then it has acceleration in the positive direction. This corresponds to speeding up if the velocity (slope of the displacement vs time line) is positive, or it corresponds to slowing down if the velocity is negative If the curvature is concave-down, like a frown, then it has acceleration in the negative direction. This corresponds to slowing down if the velocity is positive, or it corresponds to speeding up if the velocity is negative. Also, remove the word “decelerate” from your vocabulary, because it makes you think that acceleration means exclusively “speeding up”. Acceleration can mean, at least in the 1-d motion case, speeding up or slowing down.
