# Thread: gravity and energy problems

1. ## gravity and energy problems

A 920kg satellite is projected vertically upward from Earth's surface with an initital kinetic energy of 7.0x10^9J. Find
a)its max height
b)the initital kinetic energy it would have needed to keep going indefinitely
c)the inital speed it would have needed to keep going indefinitely.

which equations would I use for each part?

2. I'm not very good at helping people with math, but I'll give it a shot.

Perhaps you can use the physics formula, $\displaystyle v=\sqrt{\frac{2K}m}$ to figure out initial velocity.

The acceleration due to gravity is $\displaystyle 9.8m/s^2$. However, you can not simply use $\displaystyle d=vt + \frac{1}2at^2$ to figure out the max height because as you get farther from earth, gravity decreases.

What you need to do is use the gravitational constant, G. It is approximately equal to $\displaystyle 6.67 x 10^-11$.

Hmm, this is harder than I thought. I'll have to think about it for a while.

3. I wasn't able to get the correct answers from using the above formulas. Are there different equations I can use?