One definition of inertia is that it measures the tendensity of object that is moving in straight line to continue moving at the same speed and direction unless it is acted on by an external force. One day I was driving on an icy road. Since the road was covered with snow, I did not know the ice was under it. I decided to pass the car that was in front of me. As I rotated the steering wheel there was enough friction to make my car change lanes. But when I accelerated to pass the car, the tires just kept moving in a straight line. When I rotated the steering wheel and hit the brakes, the car slid across the road until it hit a tree. The tree exerted the external force that caused my car to stop. If you basket ball, it will continue moving until it hits the basket or someone catches it. Today I took a walk. The heat made me dizzy. When I came into the house, I fell onto the floor. The floor exerted the external force that caused me to stop. If you roll a marble across the table, it will continue moving in a straight line at constant speed until it hits a more massive object. When you are driving, you will not notice that brake fluid is leaking. When you hit the brakes, the car will continue moving in a straight line. It will not stop until it hits a more massive object. These events happened to me in the winter of 2009.