To see how equation (2.4) works, consider a ball that is dropped straight down from rest. Starting from rest means that we set vi = 0. If we ignore air friction, gravity causes a constant downward acceleration of −9.8 m/s2. This means that 9.8 m/s is subtracted from the ball's velocity each second. We can calculate that after five seconds of falling the balls velocity is −49 m/s. After ten seconds the velocity is −98 m/s, which is over 200 miles per hour! Notice that in this example, negative acceleration causes the speed to increase, because both acceleration and velocity are in the same direction. The speed increased even as the velocity became more negative!
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