Free fall with heavy box

1. Oct 7, 2009

fk08

hello,

consider a man falling from a roof with a heavy box in his hands. what should he do with the box?

2. Oct 7, 2009

A.T.

Nahh... just kidding. I would push it downwards and thus accelerate myself upwards shortly before impact.

3. Oct 7, 2009

fk08

but why shortly before impact?

4. Oct 7, 2009

Staff: Mentor

Because (assuming that the box has the same drag coefficient as the person) that will maximize the energy lost to drag. Of course, it will have to be a very tall building for drag to matter much.

5. Oct 7, 2009

fk08

consider that case without drag

6. Oct 7, 2009

Staff: Mentor

Then the timing does not matter.

7. Oct 7, 2009

A.T.

I think timing does matter without considering drag.

Think of the extreme case: You are strong enough to give the box so much momentum downwards, that you stop falling just before impact. Applying the same momentum to the box at the begin of the fall will just make you fly higher and fall even deeper.

8. Oct 7, 2009

Staff: Mentor

Good point. I think you are right, but I would have to work it out to be sure.

9. Oct 7, 2009

A.T.

Here a simulator (the box is just like rocket fuel):
http://lander.dunnbypaul.net/

10. Oct 7, 2009

fk08

to minimize the "imapact velocity" one should through the box downwards as late as possible. this can be easy shown with a velocity vs time diagram and considering, that v*t is constant.

11. Oct 8, 2009

Caesar_Rahil

ya..i think you should drop it as late as possible assuming you have enough strength in the end. I take it that you have nothing but the box, so once you come to stop momentarily after throwing, you should really make sure you don't fall long enough

12. Oct 8, 2009

Count Iblis

You can exert a far larger force using your leg muscles than using your arm muscles. While in free fall, you should put your feet on the box and jump at the last moment.

13. Oct 8, 2009

fk08

It would be interesting to calculate the force that is needed to slow down the velocity to zero.

14. Oct 9, 2009

vibjwb

15. Oct 9, 2009

A.T.

Sure, humans are to weak. It is hard enough to jump off the ground with the massive earth as support. Pushing down a box or even an elevator cabin might give you enough force to cancel out the gravitational acceleration for a moment, but you would need much more to achieve an upwards acceleration that would slow you down significantly.