# EK Physics Lesson test 3, discrete

#### aa4

##### Member
10+ Year Member
can someone please explain to me why the answer is b. i would guess it to be d, g decreases to 0, wouldn't the PE have to as well (PE=mgh)

thanks

A rocket is launched from earth to explore our solar system and beyond. As the rocket moves out of the earth's atm and into deep space the gravatational constant g decreases and approaches 0, and the gravatational potential E of the rocket:
a) also decreases and approaches 0
b) continually increases
c) remains constant
d) increases at first and then decreases and approaches 0

#### moocow012

##### New Member
10+ Year Member
PE = mgh refers only to the PE within the earth's gravitational pull. This PE is commonly used because many questions refer to objects within a gravitational field. But In general, PE increases with increasing height, so even if you left earth's gravitational pull, PE will continue to increase. Hope that helps.

#### estairella

##### Senior Member
10+ Year Member
moocow012 said:
PE = mgh refers only to the PE within the earth's gravitational pull. This PE is commonly used because many questions refer to objects within a gravitational field. But In general, PE increases with increasing height, so even if you left earth's gravitational pull, PE will continue to increase. Hope that helps.
To add to that, technically, you can never leave earth's gravitational pull, until you are infinitely away from it.

Like he said, PE = mgh is an approximation. Imagine an isolated universe with 2 balls of some mass. No matter how much you move them away, there will always be the attractive force of gravity. And because there is always a net attractive force, no matter how small, the farther away you separate them, the more potential energy they have.

#### DrBowtie

##### Final Countdown
Moderator Emeritus
10+ Year Member
The formula is U=-Gmm/r
As r-> infinity it gets closer to zero aka increases.

Enjoy!

OP
A

#### aa4

##### Member
10+ Year Member
thanks for the help. makes more sense now

#### Dart1516

##### Junior Member
10+ Year Member
5+ Year Member
aa4 said:
thanks for the help. makes more sense now

You know EK has a message board right? Just thought I'd put that out there if you don't know. It stores old questions which have already been asked and the editor of the prep books responds to your new questions.