HandsOnPhysics


Physics

 Introduction  
Speed itself is not measured directly, by calculated from a combination of time and distance measurements. Speed is simply the ratio of position change to time needed for that change, or the distance traveled each second. Vehicles in a transportation system do a lot of changing speed. They start, they speed up, they travel at constant speeds, they slow down, and they stop. Vehicle speeds in general can be very complicated. However, complicated motion can often be approximated with a combination of constant speeds and steady speed changes. Constant speeds are quite easy to deal with. Constant speed is determined by measuring the time required to travel some predetermined distance or by measuring the distance traveled in some predetermined time. If time intervals are predetermined, time is called the independent variable with distance the dependent variable (it depends on time). If the distance intervals are predetermined by the experimenter then the role switches, with time becoming the dependent variable. Which ever technique is used, the speed is still the ratio of distance traveled to time elapsed. Changing speed may be approximated with successive "constant"speeds. (This is a standard trick physicists use.) While the speed of an object is changing, we sense that the speed is never really constant, so we think of each "constant" speed as an average over some short time interval. When speed is increasing, the actual speed will be less than average at the beginning of the time interval and more than the average at the end of the time interval. When the change is speed is steady, we say the acceleration is constant. When the speed change is steady, the "instantaneous" speed in middle of the time interval turns out to be the same as the average speed over the whole interval. By determining a bunch of average speeds, the pattern of changing speed may be seen.