A favorite past time on the David Letterman show is to throw watermelons off the roof of the Ed Sullivan theater. Let us determine the height of the building from the time T of releasing the watermelon to hearing it splatter its contents on the street. Assuming T = 2.9s. What is the height h of the building?

We can either begin with the memorized constant acceleration equation

or start with the 2nd law of Newton’s laws of motion

Neglecting drag, the only force acting on the watermelon is gravity

so

Note that the acceleration is positvie because we chose the y-axis to be positive pointing down toward the street. So the equation of motion is

Integrating to get velocity

Since at t = 0, ẏ = 0, C₁ = 0 and therefore

Integrating to get position yields

Since at t = 0, y = 0, we get

(1)

Although this part is straight forward, the trick for this problem is to incorporate the time needed for sound to travel from the street to our ears on the roof. Because T is the total time, it includes the t in eq. 1 and the time for sound to travel t_s

Assuming the speed of sound v_s at 20^oC is 343m∕s, the height traveled in the time t _s that must be subtracted off is

so

So t in eq. 1 is then

Therefore, the height (y = h) is now determined from

(2)

Expanding

Collecting terms

Now we can use the quadratic formula to solve for h

with the following constants

Substituting these values into the quadratic equation yields two solutions,

and

To determine the correct answer we need to plug each solution into our time equation to see which one makes sense

For solution 1, we get

and for solution 2, we get

Clearly, since our total time is T = 2.9s, only the first solution is correct and the height of the building is

Note, that if we ignored the speed of sound, we would use the equation

which is only off by 0.7%.