The discussion centers on the concept of gravitational potential energy (Ep) and the need to account for the variation of gravitational acceleration (g) with altitude. It highlights that the simple equation Ep = g * m * z is only accurate over short distances, as g changes significantly at higher altitudes. Participants suggest using integration to calculate potential energy accurately over varying distances, with the integral ΔU = ∫(GMm/r²)dr being a key point of discussion. The conversation also touches on programming approaches to calculate gravitational potential energy over large distances, emphasizing that while calculus is necessary for deriving the equations, simpler methods can be employed for practical calculations. Overall, the thread illustrates the complexities of gravitational physics and the importance of precise calculations in programming applications.
#1
JPC
204
1
hey
Ep = g* m * z
z = (z1 - z2)
but, apparently that can only work for a short distance z value , because the value of g changes with altitude
So , is there any equation, where the change of the g value with the change of altitude is taken acount of ?
But you sure that's the equation, it sounds too simple
the thing u found is a bit like :
Ec = mgz = GMmz / z² = GMm / z
but i mean , like during the fall of the object, the value of g changes
like if you have an object 500 miles over the Earth's surface ;
during the fall, the g value won't be the same
g at 500 miles over surface isn't the same as g at 300
see what i mean ?
But you sure that's the equation, it sounds too simple
the thing u found is a bit like :
Ec = mgz = GMmz / z² = GMm / z
but i mean , like during the fall of the object, the value of g changes
like if you have an object 500 miles over the Earth's surface ;
during the fall, the g value won't be the same
g at 500 miles over surface isn't the same as g at 300
see what i mean ?
Note that g=GM/R2, where here, R is the radius of the earth. This is the acceleration due to gravity on the Earth's surface. As you move off the surface, the value of g changes, since R is now not simply the Earth's radius.
As Hootenanny says, to obtain the potential, one would integrate the expression, between the limits of the distance (however, I think he missed a "m" from the integrand!)
As Hootenanny says, to obtain the potential, one would integrate the expression, between the limits of the distance (however, I think he missed a "m" from the integrand!)
Indeed I did, good catch, you've just saved a hell of a lot of confusion later on...
#7
JPC
204
1
hey but what does the kind of sloppy sign mean ?
im only in grade 11
hey but what does the kind of sloppy sign mean ?
im only in grade 11
and what's d in the equation ?
The equation I gave you involved an integrand, which involves calculus, which you clearly haven't met yet. In that case could you post the exact problem you are trying to solve then we can all see what's what?
#9
turdferguson
312
0
Youre thinking is very sound having never encountered calculus. The simple equation for PE is only a rough approximation close to the Earth's surface. With calculus, were able to find exact answers to problems where gravity acts over long distances with varying force.
Basically, the squiggle is a symbol for integration, or finding the "area under the curve." For a straight line like y = 3, the area under the curve is the base times height. W=Fd, so if a constant 3 N force acts over 2 m, the work done is 6 J. However, calculus enables us to find the exact (no estimate) work done when force is not constant. The d before the r means that r is the variable changing. You can easily find the area under a certain section of a parabola for example.
Youre thinking is very sound having never encountered calculus. The simple equation for PE is only a rough approximation close to the Earth's surface. With calculus, were able to find exact answers to problems where gravity acts over long distances with varying force.
Basically, the squiggle is a symbol for integration, or finding the "area under the curve." For a straight line like y = 3, the area under the curve is the base times height. W=Fd, so if a constant 3 N force acts over 2 m, the work done is 6 J. However, calculus enables us to find the exact (no estimate) work done when force is not constant. The d before the r means that r is the variable changing. You can easily find the area under a certain section of a parabola for example.
IMHO, this is not the correct place to attempt to teach the OP calculus. I think that a sort of hand-waving explanation here will just further confuse the OP.
It is far better to leave the discussion of calculus, ask the OP to state the specific question he was having trouble with, and then help him with the question using the mathematics that he understands.
#11
storygeek
7
0
Refresh on energy principle
The gravitational potential energy near a planet is then negative, since gravity does positive work as the mass approaches.
Why is the potential neg near a planet? It is because the work taken to push an object out into space is positive?
BTW, JPC, are you asking the question for homework or for your curiosity?
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#12
JPC
204
1
Ok
i see a bit your general idea for calculus , but yeah , still a lot of confusion
Just wondering , Calculus is learned in what grade , or how many years after you finished school ?
My general idea was to build a program , which would be able to calculate the gravitational potential energy with big distances
But , maybe i can do that with a kind of aprox way :
with a 'Do until loop'
like the program , cuts the distances in many short distances, and does them all one by one with the general Ep = mgh equation
and then add them all up
But , i was thinking, that maybe i could make my program give exact results, so that's why i posted here
I'm from the UK, so I don't know about US grades, but I first learned calc in Year 12 (16/17 years old). If this isn't homework then we can give you a lot more help than we have been doing. Unfortunately, I don't have much time to help you now (12:30 AM in the UK); but I'm going to recommend to the mentors that this be moved to general physics where it should receive more attention.
U=-GMm/r is all you need. It is very accurate and does not need any knowledge of calculus.
To make it absolutely clear (hopefully): you do not need to know calculus in order to use this formula. You do need to know calculus in order to derive it and understand where it comes from, which is what the discussion above, involving an integral, was about.
Note that r is the distance from the center of the earth, not from the surface of the earth!
"Change", e.g. the difference in potential energy between two positions.
#19
planish
33
0
JPC said:
... My general idea was to build a program , which would be able to calculate the gravitational potential energy with big distances
But , maybe i can do that with a kind of aprox way :
with a 'Do until loop'
like the program , cuts the distances in many short distances, and does them all one by one with the general Ep = mgh equation
and then add them all up
But , i was thinking, that maybe i could make my program give exact results, so that's why i posted here
I recall a listing for a BASIC program that I was able to adapt for my Sinclair ZX80 lo, these many years ago. It was the "Moon Lander" game that involved the player trying to determine how much fuel to burn in successive burns so as to land with less than some maximum vertical velocity. If you burned too much too often, you go up or hover too long, run out of fuel, and then crash and die. If you did not burn enough, you would land too hard and crash and die. Every time you executed a burn, the combined mass of the lander reduced, so the next burn might have to be shortened.
I think that version was a simple text display of the velocity and altitude, so it should be easy to extract the routine than did the calculation. The ZX80 only had something like 4K or RAM, and you had to type this listing in, so it should be very short, very simple, and easy to deconstruct. If you peruse a couple of '80s vintage tutorials on BASIC (maybe for Apple II, Commodore 64, TRS80, Timex/Sinclair, early IBM PC, etc.) you would probably find some version of it.
Waiddaminnit - Here's some stuff I scooped from the source of a page with a Java applet that emulates the TRS-80's BASIC version of it, and they call it "Lunar Lander 2.0". Clicking on the "open program window" button gives you this:
Code:
1 PRINT"WELCOME TO LUNAR LANDER 2.0"
6 H=45
7 S=0
8 B=0
9 F=100
10 X=1
20 PAUSE 2750
30 CLS
35 A=1
40 SET(A,46)
45 A=A+1
50 IF A<127 GOTO 40
60 SET(116,X)
100 PRINT"YOU HAVE";F;"GALLONS OF FUEL"
110 PRINT"YOU ARE";H;"MILES HIGH"
120 PRINT"YOU ARE GOING";S;"MPH"
130 INPUT"HOW MUCH FUEL DO YOU WANT TO BURN?";B
131 RESET(116,H)
132 F=F-B
134 S=S+16
136 S=S-B
138 H=H-S
140 B=0
145 X=45-H
150 IF F<1 GOTO 1000
160 IF H<1 GOTO 1150
165 CLS
170 GOTO 35
1000 PRINT"OUT OF FUEL!"
1005 PAUSE 1900
1010 GOTO 10000
1150 IF S>3 GOTO 1500
1160 PRINT"LANDED SAFELY!"
1165 PAUSE 1900
1170 GOTO 10000
1500 PRINT"CRASH!"
1505 PAUSE 1900
1510 GOTO 10000
10000 CLS
10001 P=2
11000 INPUT"WOULD YOU LIKE TO PLAY AGAIN(1=YES/2=NO)";P
12000 IF P=1 THEN GOTO 1
13000 PRINT"GOODBYE"
"1150 S=3" appears to set the maximum landing velocity above which you die.
I think "SET" turns on a white pixel, indication the position of the lander.
