Lets pretend a 200 pound man is carrying two 10 pound sacks. He comes to a bridge that has a 210 pound maximum limit.

 

Assumming the bridge will fail with his 220 pound load, the man attempts to cross the bridge by tossing one sack into the air and at the point of catching the sack, he throws the second sack up. He repeats the process until he is across.

 

Essentially, the man is never actually carrying more than one sack at a time by what I guess could be described as juggling.

 

Would the gross weight of this load be that of his own weight plus that of the one sack he is carrying, or does the effect of tossing the sack in the air act as a downward force and as far as the bridge is concerned, real weight?

 

 

 

The bridge doesn't have a weight limit, it has a force limit, and F=ma. However, since the man is not a rigid body, when he caught it his arm would give thus reducing the force of impact on the bridge, so this scenario might work.

it depends how hard the throws the sacks up in the air.

That's the thing. The force required to launch the 10lb sack upwards would be considerably more than 10lb (which is the upward force required simply to stop it moving down).

Let's not forget resultants...

Originally posted by Glider

That's the thing. The force required to launch the 10lb sack upwards would be considerably more than 10lb (which is the upward force required simply to stop it moving down).

 

actually they would be the same. if he merely applied the equal and opposite force to the sack's weight, it would carry on in that direction at that velocity. it depends on how sharply he throws it, if he swung it from under his legs in a wide arc, he could get alot more speed into it with less force (though more time) than if he threw it up sharply. same goes for catching it.... if he slows it doen over a long period, then less force, then if he stood ramrod straight and let the thing slap him in the face.... watch cricketers catch cricket balls, you'll see what I mean.