Hit to the ground


What happens if an object –may be a watch or a telephone- falls down and hits the ground?

It is possible to estimate the enormous acceleration with nearly no equipment, but a little bit of calculation.


It is interesting to learn about these high accelerations that happens day after day in our life.


Very interesting for physics is, how to estimate.

May be, we only teach to find exact solutions. In science it is useful to estimate and to know something about the quality of the estimation.


What we need

-one ball of “Play Dough”,

-one ruler 1m

-one small ruler


The idea / definitions

Let the ball with the diameter D fall down on the table.

The diameter d of the circle that deforms the ball contains information about the average acceleration.

Attention! To important words: average and estimation.

-H: height before the fall, measured between table and lower edge of the ball

-h: see the sketch


If d<<D, then D is (nearly) the same before and after the crash with the table.


Velocity of the “first touch”

, simple but not exact for the following calculation.

The ball falls through the distance H+h, and these energy works in the deformation.

If the ball is not very small and not very soft, the mistake may be irrelevant.

But it is always important to think about it.