Two Balls Dropped: Separate or Together?

[darkSun]

Hello,

In this problem: http://www.physics.harvard.edu/academics/undergrad/probweek/prob1.pdf

Two balls are dropped, one on top of each other. When figuring out how high the top ball bounces, the solution assumes that the bottom ball hits the ground first, without the other ball on top of it.

I thought that ideally the two balls would stay on top of each other, since they experience the same acceleration, and would both "hit" at the same time. Is something wrong with my thinking?

 

[tiny-tim]

I thought that ideally the two balls would stay on top of each other, since they experience the same acceleration, and would both "hit" at the same time. Is something wrong with my thinking?

Hello darkSun!

No, you're right …

but it doesn't make any difference to the result if we split the equations into two stages …

and since that's easier, that's exactly what we do!

(technically, of course, the balls deform, and the top of the lower ball doesn't "know" that the bottom has hit the ground until shortly afterwards!)

 

[darkSun]

Oh, I see!

It really doesn't make a difference? Seems weird, but I'll try that.

Thank you for the help tiny-tim.