Hopefully you have now noticed these two relationships
Graphs H and I form the two "halves" of a parabola that opens upward. They represent situations in which the acceleration is positive.
Graphs J and K form the two "halves" of a parabola that opens downward. They represent situations in which the
acceleration is negative
.
Remember that acceleration can be calculated as the slope of a velocity-time graph. The velocity graphs that correspond to Graphs H and I have positive slopes. While those corresponding to J and K have negative slopes.
Whenever an object is losing speed, its velocity graph moves towards the x-axis (where velocity = zero). When it is gaining speed, its velocity graph moves away from the x-axis to values on the y-axis that represent "greater" speeds.
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