Find the time of flight?

Question

A tennis ball rolls off the edge of a tabletop of height 0.770m above the floor and strikes the floor at a point a horizontal distance 1.50m from the edge of the table. You can ignore air resistance. Find the magnitude of the initial velocity.

Answer 1

The ball moves in teh vertical (y) direction independently of what is happening in the horizontal (x) direction, so you can find the time it takes to hit the ground from:

y = 1/2at^2 ---> t = sqrt(2y/a)= sqrt(2*0.77m/9.8m/s^2) = 0.396 s

In that time, the ball moves a distance in x = 1.5 m at a constant speed in x. So,

x = vt ---> v = x/t = 1.5m/0.396s = 3.78 m/s

Answer 2

A friend of mine had this experience. The old Stapleton airport in Denver was a hard place to land because of the winds in the mountains. My friend said that when landing there, in bad weather, the plane hit the ground and bounced back in the to come down hard again. When the plane was on the ground and taxing to the terminal the pilot came on with this comment "Ladies and gentlemen we have just landed at Denver's Stapleton airport, twice."