Periodically, I use bathroom scales for experiments.  (However instead of calling them scales, I prefer the term forceometer, because they are very good at measuring the gravitational force of attraction between the earth and an object).

I’ve found that I cannot leave forceometers lying around in the workshop when kids (particularly boys) are present.  They cannot avoid the temptation to jump on them to see if they can “peg” the needle.  I can’t fault them too much though, because they are doing good empirical research.  And what they find is that it really is not all that hard to have a “weight” over 300 pounds for a very brief time.  It’s all in the acceleration.

Another place to see this phenomenon is with the stomp rocket.  Students learn very quickly that there is a direct correlation between force of stomp and distance the rocket travels.  What they may not realize that they are simply following Newton’s first law to generate a large force by maximizing acceleration.

F = ma

The above formula tells us that force is proportional to acceleration.  Acceleration is change in speed, therefore the faster an object’s (or boy’s) speed changes, the larger the force.

In the picture below, the boy is going to be accelerated by gravity as he free-falls over the entire distance that he falls. Then he is going to lose all his speed over a very short distance, so his acceleration will be much higher than gravity.  (we’ll talk more about g force in a separate post)

If the student were to simply stand on the stop rocket pad, he would exert a force equal to his own weight.  With a few calculations, we can estimate how much extra force he will generate by falling from a height of 40 cm.

Knowing that gravity will accelerate him downward at a rate of 9.8m/s per second, it is possible to determine that his velocity just before he hits the stomp rocket pad will be about 2.8 m/sec.  When he hits the stomp rocket pad, he will lose all that velocity while traveling only about 7.5 cm, which would take about 6/100 of a second.  So his stopping acceleration would be 21.23 m/sec per second, which is a little more than twice normal gravity, so he would exert a force of twice his weight by simply free-falling 40cm.

Everyone knows that stomp-rocketers won’t just free-fall; at the last moment, they’ll forcefully extend their legs to stomp, which cuts the acceleration time in half or a third so that the total force exerted would be four or even six times his own weight.  For a 50 lb kid, that could mean a total impact force of 300 pounds, which is enough to peg the needle on my forceometers (bathroom scales).  And that is why I’ve learned not to leave the forceometers lying around in the workshop, and instead make sure I have plenty of stomp rockets on hand!