# Homework Help: Homework Question, a bit stuck.

1. Oct 12, 2012

### MarkB4077

The question is: Henry is bouncing on a trampoline, using the given data below, calculate the height at which Henry will bounce to from his lowest position.

Lowest position: -0.50m
Henry's Mass: 75kg
upward acceleration: 13.3 m/s^2
upward force: 1750N
downward force: 750N
Net force: 1000N
acceleration/deceleration due to gravity: 10 m/s^2
Maximum height = ?

2. Oct 12, 2012

### Simon Bridge

Welcome to PF;
Can you show us where you got stuck?
The way to get the best out of these forums is to attempt the problem and then we can figure out where best to give you a hand.

eg. have you tried treating the trampoline as a spring?

3. Oct 12, 2012

### MarkB4077

Thanks for the reply, for me it seems every formula I try either involves knowing the time it takes or the initial and final velocity. I can't seem to find any that go solely on force and acceleration AND provides the height/distance the question requires me to give.

So really I am trying to find any formulas that I may have missed or not known about. The text book we use is absolutely worthless, so that's no help.

4. Oct 12, 2012

### Simon Bridge

Well you are making a mistake looking for equations and formulas to start with.
For problems like this you need to use your knowledge of physics to make up your own formulas.

In this case - try conservation of energy and the spring-force laws.
Imagine the person is bouncing on a big spring.
When the spring is compressed, it stores energy. When that energy is released - it turns into another kind of energy that you know a relation for that uses height. You also know a relation for how far a spring is compressed and the force the spring exerts.

If you use those relations, and the other laws you know, you can construct the correct equation that you need without looking anything up.

That is pretty much what the problem wants you to learn to do.

5. Oct 12, 2012

### MarkB4077

Thanks Simon, you made a lot more sense of that than the teacher did, I think I'll get it now.

6. Oct 13, 2012

### Simon Bridge

OK - let me know how you get on.