 Now, let's work through some exercises. Let's think about the effect that uniform circular motion has on people. So let's imagine somebody standing at the equator on Earth. The Earth is rotating about its axis once every day. What is the acceleration of a person standing at the equator due to the Earth's axial spin? To solve this, we need to approximate the as a circle for this problem so we can use our uniform circular motion tools. We know that the centripetal acceleration is equal to the tangential velocity squared divided by the radius, and we know that the speed is given by 2 pi r divided by big t, the period. If we substitute in our expression for the speed into our expression for acceleration, we find that the centripetal acceleration is 4 pi r on big t squared. So now we can substitute values in. The radius of the Earth is about equal to 6400 kilometers, and the period of the rotation of the Earth is one day or 24 hours. But it's generally good to give answers in normal SI units, so we're going to convert kilometers into meters and hours into seconds. One kilometer is 1000 meters and one hour is 60 minutes, which is itself 60 seconds, so one hour is then equal to 3600 seconds. So in our expression, replacing kilometer with 1000 meters and hour with 3600 seconds, we find that the total centripetal acceleration of a person standing on the surface of the Earth at the equator is 0.034 meters per second squared. This acceleration comes from the gravitational force of the Earth. Now we're going to try a related problem that's a bit more complicated.