This is the question: During a time interval of 5 seconds, an object moves through a half circle with a radius of 10 meters, as shown above. What is the magnitude of the object's velocity during this motion? Note: I do not know how to post the diagram, but it simply shows a curved arrow going around a half circle, with the object ending exactly across from its starting point. I solved this using V=2(pi)(r)/Period. The period of half of a circle is five seconds so the period of a full circle is ten seconds. So: 2pi(10)/(10 seconds) I got an answer of 2 pi m/s. The book says the answer is 4 m/s because the change in displacement is twenty meters (diameter) and the time is five seconds, so d/t=20/5=4 m/s. Who went wrong?