Very large accelerations can injure the body, especially if they last for a considerable length of time. one model used to gauge the likelihood of injury is the severity index (si), defined as si=a5/2t . in the expression, t is the duration of the acceleration, but a is not equal to the acceleration. rather, a is a dimensionless constant that equals the number of multiples of g that the acceleration is equal to. in one set of studies of rear-end collisions, a person's velocity increases by 12.0 km/h with an acceleration of 37.5 m/s2 . let the +x direction point in the direction the car is traveling. what is the severity index for the collision