Question

A jetliner, travelling northward, is landing with a speed of 69 m/s. Once the jet touches down, it has 750 m of runway in which to reduce its speed to 6.1 m/s . Compute the average acceleration (magnitude and direction) of the plane during landing.

Answer 1

We organize ourselves by drawing a picture of the landing plane, as shown in Fig. The plane touches down at x = 0; that’s where the motion begins, as far as we’re concerned. The initial velocity is v₀ = 69 m/s . In the final position (after it has travelled the full extent of the runway), x = 750 m and v = 6.1 m/s . But we are not given the time t for this motion to take placed and we don’t know the (constant) acceleration a. 

If we want to get a we can use the below eqn. because it doesn’t contain the time t. Plugging in the numbers, we get

We get a negative answer, and we expect that; the plane’s velocity (in the direction of motion, North) is decreasing. The acceleration has a magnitude of 3.15 m/s² and its direction is opposite the direction of motion, i.e. South.