One could be traveling north and the other south. Look at Fig. 5.3 in text. We use boldface to indicate vectors or arrows over the quantity. Look at Fig.
Acceleration can be due to a change in speed or a change in direction.
Circular motion at constant speed is called centripetal acceleration . Galileo is generally known as the father of physics. He was the first to express many of our current ideas, for example inertia . Inertia is the property of a body that causes it to remain at rest or in motion unless acted upon by an outside force. Galileo actually did experiments to show that bodies behaved this way.