Just watch the MIT 8.01 Lec 2. Does anyone know how how to figure out how many balls you should see in the strobe picture (referring to the second alertness test). Showing it mathematically.

Question

anonymous · Answer

Well the ball will take a finite amount of time to reach the floor, which can be found using the equation$$S = ut + 1/2at ^{2}$$S is the displacement. 'ut' can be disregarded as there is 0 initial velocity (u). a = g (9.8 m/s/s). Solving this for t you should get two answers, one of which will be irrelevant (a negative time). If you then divide the time obtained by the period of the strobe,$$period = 1\div frequency$$ you will be able to find how many flashes occur during the falling of the ball, and thus how many times you should be able to see the ball.

anonymous · Answer

thank you so much!! 

okay so i got about 0.78 for the time and then using your method came to the conclusion that you should see the ball about 7.8 times

anonymous · Answer

:) you're welcome! but bear in mind that you can't see it 7.8 times, you'll see it 7 times with 0.8 of a second split between either end depending on when during the fall the first flash occurs :)