1. Oct 11, 2005

404

I was doing some momentum questions. There some that goes like "a man dives into water, and comes to a stop x seconds after reaching water, what force did the water act on him" or "stuntman jumps onto a mattress, matress compressed down x meters before he is brought to rest, what force did the matress apply on him?"
Basically, answer= (guys mass)(acc. of medium) + 9.8(guys mass)

Everytime I do those questions, I keep forgetting account for gravity, and I don't know why you are suppose to add 9.8*(guys mass). I mean, isn't gravity pulling the guy down, while the water/mattress's force is pushing the guy upwards, to stop him? So why do you add the 2 forces, if anything, wouldn't you subtract? (someone please clear this up for me )

2. Oct 17, 2005

Tom Mattson

Staff Emeritus
Sorry for the late reply. In case you're still interested, here are my comments on this.

The matress supplies an upward force that has to do 2 things:

1.) Decelerate the man.
2.) Support his weight.

So if we apply Newton's second law to the man + mattress system we have:

$$\Sigma F=F_{mattress}-mg=ma$$

So you are subtracting the two forces. And as you can see, upon solving for $F{mattress}$ you get $mg+ma$.

3. Dec 1, 2007

Physwiz

here's another way of looking at it.....

downward force= (mg+ma)....this is applied onto the surface by the guy

for every force applied to the surface(mattress/water).....there will be an equal and opposite reaction by the surface in the upward direction....