Feynman

Please let me cross the bridge before you cross it.

Juggling them means that he has to throw them upwards.

The bigger the bundles, larger the movements he has to make and the longer the bundles have to remain into the air.

The longer the bundle have to remain into the air, the faster he has to throw them upwards.

the faster he has to throw them upward, the more reaction will be exerted by his throwing them of into the air.

I.e. the very act of throwing them into the air increases the force he exerts on the bridge (his weight plus the downward reaction from the accelerating of the bundle upward).

I won't get into calculations, but my intuition is that the average weight increase caused by the accelerating the three bundles to launch them into the air is exactly the weight of the three bundles. Assuming you would have constant acceleration and zero delay between the load switch. Accounting for friction and other losses and inefficiencies, it is actually more. And if you have the slightest delay between switching of the loads, the peak reaction will be more than three times the weight.





Here's a physics teaser that I never tried to solve, (nor anybody I know).

You have a fly resting inside a sealed large jar, the jar is on a sensitive balance.

If the fly starts to fly inside the jar (assume constant-altitude flight), what will the balance register ?

Will the reading on the scale vary according to the fly's altitude (methink yes, because of a ground effect; and it will probably take place if the fly is at an altitude lower than her wingspan).

Otherwise, is all the momentum imparted to the air lost into vortices that have no major vertical component ? (i.e. the weight registered will be that of the empty jar minus and all air momentum will be converted to heat).

I entirely forgot my fluid mechanics...

Anybody who ever tried the experience, come back to me.