# Homework Help: Speed, Kinetic Energy, Force Problem.

1. May 1, 2007

### marckc22

1. The problem statement, all variables and given/known data
An 80 kg man jumps from a height of 3.00 m onto a platform mounted on springs. As the springs compress, the platform is pushed down a maximum distance of 0.200 m below its initial position, and then it rebounds. The platform and springs have negligible mass.

a. what is the man's speed at the instant the platform is depressed 0.100 m

b. if the man had just stepped gently onto the platform, how far would it have been pushed down?

2. Relevant equations
a = V/t
v = d / t
F = ma

3. The attempt at a solution
i've tried solving this one, and im stuck. heres what i did:

a = V/t
V = at

V = d/t
d/t = at

t = (d/a)^-2 = 0.553 s

V = 9.81(0.553) = 5.425 s

F = ma = 80 kg (9.81) = 62.784 kJ

Last edited: May 1, 2007
2. May 1, 2007

### danago

Whats the actual question?

3. May 1, 2007

### marckc22

i edited my previous post. i added the questions. my bad. :(

4. May 1, 2007

### PhanthomJay

Although this problem may be solved using kinematics and newton's laws, it is much easier to use energy methods. Are you familiar with the law of energy conservation?