UP1 Coursework, force problems.

[asteadydeclin]

Homework Statement

A 20kg child jumps in the air to try to shoot free throws while his older sister is practicing basketball. His center of mass is normally 30cm off the ground. The lowest he can get his center of mass is 20cm off the ground, and the highest he can get it before he can no longer push is 40cm. He can push against the floor with a force 3.5x his own weight.
a) If you identify just the child as your system, what is the net external force on the child as he is jumping?
b) Assuming the child was at rest when he started pushing on the floor, what is his kinetic energy when his feet leave the floor?
c) How much work did the floor do on the child?
d) How high can he jump? (May want to change your system here.)

Homework Equations

This is the main part I'm having problems with. I have no idea which equations to use. Our professor was out all last week after surgery, and the TA instructing the class was very hard to understand. I copied down the example problem from class, and the four equations used to solve it, but my windshield decided to stop holding water and ruined my notebook. If you could at least put me on the right track with the right equation(s), I would be more than grateful. Thanks in advance.

The Attempt at a Solution

N/A

 

[Delphi51]

It says the force is "3.5x his own weight".
Can you find his weight?

 

[asteadydeclin]

It says the force is "3.5x his own weight".
Can you find his weight?

Well, yeah, the problem gives his weight. So the force would be 70N..

 

[Delphi51]

Not quite. Look up the formula for weight (force of gravity). If you can't find it in your book, look here:
http://en.wikipedia.org/wiki/Weight#Gravitational

 

[asteadydeclin]

Ohhhh, I got ya. We've never dealt with the differences between weight and mass, so typically when she says weight she means mass. Is there part of the problem that would actually require weight instead of mass?

 

[Delphi51]

It says the pushing force is 3.5 x the WEIGHT.
The units will not work out right if you use the mass in kg.
Force has units of Newtons. Weight is in Newtons, too.