# Vertical Elastic Collision Question

## Homework Statement

A 70.0 kg person steps through the window of a burning building and drops to a rescue net held 8.00 m below. If the surface of the net is 1.40 m above the ground, what must be the value of the spring constant for the net so that the person just touches the ground when the net stretches down.

## Homework Equations

How do I find the velocity of the man falling?

## The Attempt at a Solution

Eg + Ee + Ek = Eg' + Ee' + Ek'

Eg + Ek = Ee'

mgh + .5mV2 = .5kx

(70)(9.8)(9.4) + .5(70)V2 = 0.5k(1.4)2

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Doc Al
Mentor
How do I find the velocity of the man falling?
What's the man's initial speed? (He just steps through the window and falls.)

What's the man's initial speed? (He just steps through the window and falls.)
Yea steps through and falls

Doc Al
Mentor
Yea steps through and falls
So what do you think his initial speed is?

So what do you think his initial speed is?
I got 12.2

Doc Al
Mentor
I got 12.2
Is that the man's initial speed, when he just steps out the window? Or did you solve for the speed he would have at some later time?

Hint: You should be able to state the man's initial speed immediately--no calculation needed.

Is that the man's initial speed, when he just steps out the window? Or did you solve for the speed he would have at some later time?

Hint: You should be able to state the man's initial speed immediately--no calculation needed.
0 obviously I'm stating the velocity through the air

Doc Al
Mentor
0 obviously I'm stating the velocity through the air
All you need to solve this problem is the man's initial speed, not his speed at some later point. Compare initial energy to final energy.

All you need to solve this problem is the man's initial speed, not his speed at some later point. Compare initial energy to final energy.
which energies kinetic or potential?

Doc Al
Mentor
which energies kinetic or potential?
Total energy: Kinetic + potential.