First Law of Motion

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Section 6.3 First Law of Motion

First law of motion is also called the law of inertia. The law of inertia is based on experiments by Galileo Galelei. Figure 6.18 illusrates the basic setup of his experiments. A block is released from rest from the same height on the slope, but as friction on the horizontal board is reduced, the block goes farther and farther before coming to rest. He concluded that if there was no friction, the block would continue to move with uniform velocity forever. That, in essense is the first law of motion.

Figure 6.18. A block is released at rest from the same place each time so that when the block is at the bottom of the ramp, it has the same speed. When the track is polished, reducing friction, the block goes farther before coming to rest.

You may be wondering: “ What propels objects at constant velocity to be in a perpetual motion if no force were to act?” There is really no answer to this 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 with respect to inertial frames.

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