The minus sign indicates the average velocity is also toward the rear of the plane. Elapsed time Δ t is the difference between the ending time and beginning time, and end at 11:50 A.M., so that the elapsed time would be 50 min. For example, a lecture may start at 11:00 A.M. To find elapsed time, we note the time at the beginning and end of the motion and subtract the two. How does time relate to motion? We are usually interested in elapsed time for a particular motion, such as how long it takes an airplane passenger to get from his seat to the back of the plane. This allows us to not only measure the amount of time, but also to determine a sequence of events. We could then use the pendulum to measure time by counting its swings or, of course, by connecting the pendulum to a clock mechanism that registers time on a dial. We might, for example, observe that a certain pendulum makes one full swing every 0.75 s. The SI unit for time is the second, abbreviated s. The amount of time or change is calibrated by comparison with a standard. It is impossible to know that time has passed unless something changes. In physics, the definition of time is simple- time is change, or the interval over which change occurs. It may be a number on a digital clock, a heartbeat, or the position of the Sun in the sky. Every measurement of time involves measuring a change in some physical quantity. As discussed in Physical Quantities and Units, the most fundamental physical quantities are defined by how they are measured.
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