Homework Help: Force Exerted From the Ground During a Jump.

1. May 24, 2012

Hello! This problem has been driving me crazy all week long, and now I am down to the last few hours before the assignment is due. Any help would be greatly appreciated!

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

An average person can reach a maximum height of about 60cm when jumping straight up from a crouched position. During the jump itself, the person's body from the knees up typically rises a distance of around 50cm. To keep the calculations simple and yet get a reasonable result, assume that the entire body rises this much during the jump.

Part A required me to find the initial speed a person would need to reach a maximum height of 60cm, for which I correctly answered 3.4m/s.

Part B is where I find myself stuck. It asks, "In terms of this jumper's weight (w), what force does the ground exert on him or her during the jump?"The answer will read:
F= ___ w.

2. Relevant equations

ƩFy=may
w=mg
...

3. The attempt at a solution

Honestly, I've just been trying to figure out how I am supposed to get anywhere without first knowing the mass of the person jumping. I suspect it has something to do with the initial speed I had calculated for Part A being related to an opposite force, coming from the ground going to the person jumping.

Last edited: May 24, 2012
2. May 24, 2012

ehild

The body accelerates from zero to 3.4 m/s speed during the first stage of the jump, while rising 0.5 m. Determine the acceleration. From the acceleration, you can find the force.

ehild

3. May 24, 2012

Staff: Mentor

You are asked to express the answer in terms of the jumper's weight. So given the weight as W, what's his mass?