a). The Earth travels around the sun in a path that's nearly a circle, with a radius of roughly 93 million miles.
The circumference of the circle is (2 · π · radius) = about 5.843 x 10⁸ miles.
Average Speed = (distance covered) / (time to cover the distance)
The time to cover the distance is 1 year.
Speed = (5.843 x 10⁸ mile/year) (year / 365 day) (day / 24 hour)
Speed = (5.843 x 10⁸ / 365·24) (mile · year · day / year · day · hour)
Average speed = 66,705 miles/hour .
b). The Earth's average velocity over a period of 1 year is zero.
Velocity = (displacement) / (time)
Displacement = (distance and direction between start-point and end-point). After one year, the Earth is located at the point where it started, so its displacement over the period of 1 year is zero.
