Solve Jumper Question Homework: Part A & B with Initial Speed & Force

[dkirov1]

Homework Statement

A person reaches a maximum height of 64 cm when jumping straight up from a crouched position. During the jump itself, the person's body from the knees up rises a distance of around 41 cm. To keep the calculations simple and yet get a reasonable result, assume that the entire body rises this much during the jump.

Part A: With what initial speed does the person leave the ground to reach a height of 64 cm?
Part B: In terms of this jumper's weight W , what force does the ground exert on him or her during the jump?

I figured out the first part, but I can't figure out the second part. :(

Also the answer to part B has to be a number? This is online homework and the answer is in the format:

F=_____W

Homework Equations

F=ma

The Attempt at a Solution

The velocity for part A was 3.5
I tried to figure out the time by using v=d/t, and got t=.117 seconds which seems wrong.
I tried to figure out the a so I used vf-vo/t=29.91?
and using 29.91 as a tried to figure it out. But I'm just stuck. I have no idea.

 

[Xisune]

The force the ground exerts on the person is going to last 41 cm, the initial velocity would be 0, the final velocity would be the answer to part a, from there you can solve for acceleration.

 

[gbaby370]

A very simple way to solve this, assuming this is a closed system, would be to use the attached formula (Egmax=Ekmax); rearrange algebraically. I would assume the force the ground exerts on her would be gravity, so F is mg or F=Wg.

Hope this helps.