Need help on a force problem

[krypt0nite]

An exceptional standing jump would raise a person 0.80 m off the ground. To do this, what force must a 70kg person exert against the ground? Assume the person lowers himself .20m prior to jumping?

Ok, i know i need to find the acceleration and use the Fnet=ma equation but where do i start first?

[Soveraign]

There are two ways to look at this problem. One way is conservation of energy, the other is kinematic.

For kinematic, you might first look at what velocity is necessary to "throw" an object straight up 0.80 m. Once you have this, then you can look at what acceleration is necessary to reach this velocity within 0.20 m.

[Sirus]

The energy method is also interesting. Consider the final and initial mechanical energy (which is conserved because this is an isolated system). Remember that E_{Mechanical}=E_{Kinetic}+E_{Potential} here.