# Homework Help: Formula to calculate newtons

1. Jan 8, 2008

### bl00d

hi, im not really sure where this belongs, and being a complete noob at physics, i was just enquiring if someone could inform em of the formula to calculate the distance an object weighing 80kg would need to fall in order to be effected by 4,400 newtons?

2. Jan 8, 2008

### olgranpappy

that doesn't make sense.

3. Jan 8, 2008

### olgranpappy

... do you mean Joules? Have 4400 joules of work done on it by gravity?

4. Jan 8, 2008

### bl00d

i didnt quite think it did, sorry about that.
im doing a project on the death penalty (not a nice topic i know), in particular hanging, and im trying to find out what an average drop distance is. i can find the amount of force that the rope needs to apply to the neck when the slack is used up (~4,400 newtons or 450kgf) and a general drop distence, but the actual distance seems to alude my search efforts.

5. Jan 8, 2008

### olgranpappy

oh. i see. you want the force when the rope snaps. you didn't mention a rope in the OP :)

you would need to know the time interval over which the rope will stop the body. if you can estimate this somehow then you can find the energy/time and get the energy from the distance of the drop.

6. Jan 8, 2008

### bl00d

yeah sorry about that, i just didn't mention it becuase i know some people become quite squeemish about such topics.
an average estimate would be at about 2-3 seconds i guess =S

7. Jan 8, 2008

### bl00d

would you happen to know the formula by the way?

8. Jan 8, 2008

### olgranpappy

yikes. sorry. I meant momentum/time... heh, wrong units.

Anyways. There are a few things involved. If you can estimate the *time interval* T over which the rope snaps then the force on the neck is given by

P/T

where P is the momentum at the bottom of the fall which is given by

P = m*Sqrt[2 g H]

where H is the distance the body fell, and m is the body's mass and g is the acceleration due to gravity (9.8 m/s^2). Use all SI units.

9. Jan 8, 2008

### olgranpappy

That is, you want

4400 Newtons = 80*Sqrt[2*9.8*H]/T

If you know the numerical value of T then you can solve for H.

H = ((4400*T/80)^2 )/19.6

10. Jan 8, 2008

### olgranpappy

P.S., T is probably a lot shorter than 2 or 3 seconds. Probably more like 0.1 seconds.

11. Jan 9, 2008

### bl00d

ah but the rope is between 5 and 9 feet in length for the long drop (to which the newtons is accurate) thats why i thought it might be longer =\

still a little flabberghasted though :s

12. Jan 9, 2008

### olgranpappy

Not the DROP time. The drop time, indeed, is about a couple seconds.

T is The STOP time--The time that is take for the rope, at the bottom of the drop, to pull tight.

13. Jan 9, 2008

### blochwave

Yah, it's not the fall that kills you

it's the sudden stop at the end ^_^

Take your fist and punch the nearest wall. Ow. Now punch a block of jello just as hard. Tee-hee. In both cases your fist went from punch speed to 0, what was the difference? When you punched the wall your fist from punch speed to 0 in a fraction of a second.

14. Jan 9, 2008

### olgranpappy

I'd recommend this only as a thought experiment.

15. Jan 9, 2008

### blochwave

I would definitely recommend punching the jello though

16. Jan 9, 2008

### olgranpappy

Bill Cosby will be pissed!