Calculating Final Velocity: An Airplane Slowing Down after Landing

Draw a sketch. We draw the acceleration vector in the direction opposite the velocity vector because the plane is decelerating.

Figure 2.28

1. Identify the knowns. $v_0=\text{70}\text{.}\text{0 m/s}$, $a=-1\text{.}{\text{50 m/s}}^2$, $t=\text{40}\text{.}0\text{s}$.

2. Identify the unknown. In this case, it is final velocity, $v_{\mathrm{f}}$_.

3. Determine which equation to use. We can calculate the final velocity using the equation $v=v_0+\text{at}$.

4. Plug in the known values and solve.

The final velocity is much less than the initial velocity, as desired when slowing down, but still positive. With jet engines, reverse thrust could be maintained long enough to stop the plane and start moving it backward. That would be indicated by a negative final velocity, which is not the case here.

Figure 2.29 The airplane lands with an initial velocity of 70.0 m/s and slows to a final velocity of 10.0 m/s before heading for the terminal. Note that the acceleration is negative because its direction is opposite to its velocity, which is positive.