One-DimensionalMotionwithUniformAcceleration(2)

　　Because both x and　are zero, the equation comes out to But don’t be hasty and give the answer as 12 m/s: remember that we devised our coordinate system in such a way that the down direction is negative, so the ball’s final velocity is –12 m/s.　　How high does the ball travel?　　We know that at the top of the ball’s trajectory its velocity is zero. That means that we know that　= 12 m/s, v = 0, and m/s2, and we need to solve for x:

　　How long does it take the ball to reach its highest point?　　Having solved for x at the highest point in the trajectory, we now know all four of the other variables related to this point, and can choose any one of the five equations to solve for t. Let’s choose the one that leaves out x:

　　Note that there are certain convenient points in the ball’s trajectory where we can extract a third variable that isn’t mentioned explicitly in the question: we know that x = 0 when the ball is at the level of the student’s hand, and we know that v = 0 at the top of the ball’s trajectory.

（责任编辑：申月月）