A car accelerates from rest when a traffic light turns green. A cup of coffee that was sitting on the dashboard of the car falls into the driver's lap. The driver could rightly argue that the cup's apparent motion was due to its inertia. It had a tendency to stay still and the car accelerated from beneath it. True or false?
False. The cup's apparent motion is not due to its inertia. According to Newton's first law of motion, an object at rest tends to stay at rest unless acted upon by an external force. In this case, the external force is the acceleration of the car. As the car accelerates, the cup remains at rest relative to the car's initial speed but it falls into the driver's lap due to the force of gravity acting downward.
