# Homework Help: Impulse and depth

1. Nov 8, 2007

### physicsbhelp

[SOLVED] Impulse and depth********!!!!!!!!!!!!!!!!!!!!!!!!

1. The problem statement, all variables and given/known data

In February 1955, a paratrooper fell 370 m from an airplane without being able to open his chute but happened to land in snow, suffering only minor injuries. Assume that his speed at impact was 58 m/s (terminal speed), that his mass (including gear) was 85 kg, and that the force on him from the snow was at the survivable limit of 1.2 105 N.

(a) What is the minimum depth of snow that would have stopped him safely?

?m

(b) What is the magnitude of the impulse on him from the snow?

? kg·m/s

2. Relevant equations

I have no clue how to do this problem

3. The attempt at a solution

I DONT KNOW. i am in desparate need of help.

ANY CLUES or ANY hints will help.

THANK YOU!!!!!*****************

2. Nov 8, 2007

### physicsbhelp

this is URGENT, I CAN USE ANYONE'S HELP IF THEY ARE WILLING TO LEND ME A HAND FOR THIS PROBLEM, PLEASE!!!

3. Nov 8, 2007

### physicsbhelp

In February 1955, a paratrooper fell 370 m from an airplane without being able to open his chute but happened to land in snow, suffering only minor injuries. Assume that his speed at impact was 58 m/s (terminal speed), that his mass (including gear) was 85 kg, and that the force on him from the snow was at the survivable limit of 1.2 105 N.

(a) What is the minimum depth of snow that would have stopped him safely?

?m

(b) What is the magnitude of the impulse on him from the snow?

? kg·m/s

ANY HELP ON THIS PROBLEM WILL BE GREATLY APPRECIATED

4. Nov 8, 2007

### Dick

Ok, so what's big panic? It's just a physics question. You have read the forum rules, right? You have to contribute first. For example, how is an initial velocity and a final velocity and a stopping distance related to acceleration? You can find a formula someplace I'm sure. Ordinarily, I would have stopped at the fourth sentence, so I'm giving you extra patience. Don't push me. We don't get paid for doing this.

5. Nov 8, 2007

### physicsbhelp

first off, im stressed because this problem is worth a lot to my grade--you should not worry about that. and don't degrade me or talk down to me.
um for your info. i have contributed. if you dont want to help me then don't, but you do not have to get mad. i don't need your patience and what do you mean by "don't push me" stop being such a jerk, im not stupid, i know that no one get paid.

6. Nov 8, 2007

### Dick

Fine, I understand you are stressed. Sorry. I'm not mad, you apologized. Now I'm just trying to help. But you haven't contributed yet, you have just posed a question without telling us what you know. Ok, I'm telling you that there is an equation that relates an initial velocity and a final velocity and a distance in between with the acceleration. Can you find it? It involves velocities squared and an acceleration times a distance. Calm down.

7. Nov 8, 2007

### aq1q

basically, he wants us to do it..

8. Nov 8, 2007

### physicsbhelp

yes, i know that equation but see for the problem you dont need to find out acceleration.
anyways, this is what you want me to do: v2-v = 2as so acceleration = 4.54 but you see the question is not asking for the acceleration

9. Nov 8, 2007

### physicsbhelp

so now what do i do?
i know that impulse = Force multiplied by time but i know neither
and also we have to find out the minimum depth. i dont know how to do eith erone

10. Nov 8, 2007

### physicsbhelp

can either of you help me now? :(

11. Nov 8, 2007

### Dick

Acceleration has a close relation with force. F=ma.

12. Nov 8, 2007

### aq1q

Im in ap physics C this year, lol i know ur pain.. but see.. life isn't fair... I joined this forum mainly cause i just love physics and i like helping ppl out, and learn from the experience. I mean, I'm a student too.
Don't expect to just join the forum and paste hw or w/e and then write in bold letters that u need immediate help. I'm trying to help u out man. Personally i dont care, but when ppl see that kind of selfishness they will ignore ur threads.. so ya keep that in mind..

ok now in that problem...

u know F=ma, u are given a force and mass, find the acceleration.. u know the velocity before the impact and the velocity after it stops (0). Also, you can assume that the person is falling exactly vertically with no velocity in the x component (very poor assumption because planes aren't stationary). That should be enuf.. show some work.
hint: look for an equation which has velocity, acceleration and distance.

if u are still confused, i suggest looking in wikipedia.org instead of yelling at us for not helping

13. Nov 8, 2007

### aq1q

hey Dick, i might need ur help for the second part. i mean, i know what to do, but see it gets a bit confusing cause i have learned that impulse= mV1-Mv2, but in wiki it says impulse is F(delta T), but i always -F(delta T). huh could u tell me which one it was again? i'm forgetting.

14. Nov 8, 2007

### Dick

vi^2-vf^2=2*a*s. Yes. But how can you say a=4.54 in whatever units? You don't know s yet. Compute a from F=ma, where F is the maximum survivable force. Put a and the velocity into that and find s.

15. Nov 8, 2007

### aq1q

yup, but Dick, did u get my post lol?

16. Nov 9, 2007

### Dick

m*v1-m*v2=m*(v1-v2)=m*a*t=F*t. Whether it's +F*t or -F*t depends on whether you are talking about the impulse transmitted from 1 to 2 or the impulse transmitted from 2 to 1. Just like forces. And like I pointed out to the OP it is rude to interject you own problem into somebody elses post.

17. Nov 9, 2007

### aq1q

lol im pretty sure... dick and i are saying the same thing.. and he didn't say that a is 4.54 and i never said a is 1411.764

18. Nov 9, 2007

### aq1q

hey physicsbhelp what is the force again.. i cant really understand is there a decimal after 1?

19. Nov 9, 2007

### physicsbhelp

ooooo okay i get it now! thakns so that is the answer to the first part of the question right? so s = 1.1914
im sry im a little slow at physics. please forgive me.
so then how do i find time because i have to ifnd impulse now.

20. Nov 9, 2007

### aq1q

i dunno cause lol i cant read the force it says "1.2 105N" why is there a space