Section 6.1 First Law of Motion
This law is also called the law of inertia. The law is based on experiments by Galileo Galelei. Figure 6.1.1 illusrates the basic setup of his experiments. A block is released from rest from the same height on the slope, but depending on the friction on the horizontal board, the block goes different distance before coming to rest. He concluded that if there was no friction, the block will continue with uniform velocity for ever. That, in essense is the first law of motion.
You may be wondering: “Why can't the velocity of an object go to zero without a force? What propels objects at constant velocity to be in their perpetual motion?” There is really no answer to these questions. Instead, we assert that nature has an inherent tendency of maintaining the motion. We give this natural tendency a name - the inertia of motion. But beware that giving a name is not the same thing as having an explanation!
Formally, the observation by Galileo was made with respect to a frame which itself is at rest with respect to the incline and the horizontal board on which motion occurs. Actually, he would have noticed the same result had he observed the ball from a car moving at constant velocity. If we imagine a “truly at-rest frame”, say at the “center of the universe”, then any frame that does not accelerate with respect to this imagined frame will give us the same result. These frames are called inertial frames. Although physics can be done with respect to any frame, but we will mostly write our formulas in inertial frames.