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Published online by Cambridge University Press: 05 June 2012
Recall the questions about motion that I listed as the “more advanced” ones in section 2.1:
Q4+Q5: Is the velocity changing with time?
Q6: If the velocity is changing, what is causing it?
Let us ask these questions of the motion of a baseball whose worldline is shown here on the spacetime diagram. The worldline has a kink at point A, the time-coordinate of which is t = 3 seconds, so we can tell that the velocity of the baseball changed at t = 3 seconds. The baseball has been hit by a bat at this point. The worldline before A is vertical, so the baseball was at rest before being hit. From the slope of the worldline after A, we can tell that the velocity of the baseball after being hit was +1 meters per second. So the answers to the above questions in this case are:
A4+A5: The velocity of the baseball changed instantaneously at t = 3 seconds from 0 meters per second to +1 meters per second.
A6: The velocity of the baseball changed because it was hit by a bat at t = 3 seconds.
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