133 consider what would happen if the body goes around the circle completely in one second. Then, if you pick two times one second apart, you would conclude the velocity isn’t changing at all, so there is no acceleration.) 20.5 An Accelerating Body that isn’t Moving W e’ve stated before that a ball thrown vertically upwards has constant downward acceleration of 10 meters per second in each second, even when it’s at the very top and isn’t moving at all. The key point here is that acceleration is rate of change of velocity. You can’t tell what the rate of change of something is unless you know its value at more than one time. For example, speed on a straight road is rate of change of distance from some given point. You can’t get a speeding ticket just for being at a particular point at a certain time—the cop has to prove that a short time later you were at a point well removed from the first point, say, three meters away after one-tenth of a second.
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