Exercise 5.4 An object has a constant acceleration of 50 mph/s. After accelerating for a time T, its velocity is 160 mph. (Note: T is a duration.) Write an algebraic expression for the initial velocity in terms of T. In our first examples of acceleration, we have worked with velocities expressed in miles per hour to make it easier to give an interpretation of the number obtained for the acceleration. But metric units are almost always used in the sciences. An acceleration of 5 mph per second would be 2.2 m/s per second in the metric system since 5 mph is 2.2 m/s. This acceleration could be written 2.2 m/s/s, read 2.2 meters per second per second." In practice, this is often written 2.2 m/s, read "2.2 meters per second squared." A square second makes no sense; m/s is just a shorthand way of writing m/s/s. It is important to realize that the two seconds in m/s/s play different roles. The second in m/s is just there as part of the velocity unit (like hour is part of the unit mph). The other second is the unit of time we use when telling how much the velocity changes in one unit of time.