discussion | summary | practice | problems |

## Gravitational Potential Energy

## Discussion

### introduction

Short bit of calculus.

U = − _{g} | ⌠⌡ | F · ds |

| r | | | |

U = − _{g} | ⌠⌡ | − | Gm_{1}m_{2} | dr |

r^{2} | ||||

| ∞ | | | |

U = − _{g} | Gm_{1}m_{2} | ⎛⎝ | 1 | − | 1 | ⎞⎠ |

r | ∞ |

and here it is…

U = − _{g} | Gm_{1}m_{2} |

r |

where…

U = _{g} | gravitational potential energy |

m_{1}m_{2} = | masses of any two objects |

r = | separation between their centers |

G = | universal gravitational constant (6.67 × 10^{−11} Nm^{2}/kg^{2}) |

Note that there is no Δ in this expression. Discuss here.

Discuss potential vs. potential energy somewhere.

What about the old equation?

**Δ U_{g} = mgΔh**

It’s hidden in the new equation.

U = − _{g} | Gm_{1}m_{2} |

r |

Let me show you.

ΔU =_{g} | | U_{f} | − | U_{i} |

| ||||

ΔU =_{g} | | U(_{g}r + Δh) | − | U(_{g}r) |

| ||||

ΔU =_{g} | − | Gm_{1}m_{2} | + | Gm_{1}m_{2} |

r + Δh | r |

Combine terms over a common denominator.

ΔU = _{g} | Gm_{1}m_{2}((r + Δh) − r) | |

r(r + Δh) | ||

ΔU = _{g} | Gm_{1}m_{2}Δh | |

r(r + Δh) |

Multiply by “one”.

ΔU = _{g} | Gm_{1}m_{2}Δh | | r |

r(r + Δh) | r |

Swap terms in the denominators.

ΔU = _{g} | Gm_{1}m_{2}Δh | | r |

r^{2} | r + Δh |

Factor some stuff out of the numerator.

ΔU = _{g}m_{2}h | Gm_{1} | | r |

r^{2} | r + Δh |

Do you see it? If *r* is the mass of the Earth, *m*_{1} is the mass of the Earth, and *m*_{2} is the mass of something being lifted, then…

g = | Gm_{1} |

r^{2} |

is the acceleration due to gravity on the Earth’s surface. Making this substitution (and dropping the subscript, since we only have one mass left), we get…

ΔU = _{g}mgΔh | r |

r + Δh |

The first part of this expression is our old friend, the original equation for gravitational potential energy. The second term is a correction factor. For ordinary heights, this term is essentially one. Let’s confirm this using a really high height — the top of the spire on the Burj Khalifa in the United Arab Emirates (818 m).

r | = | 6,371,000 m |

r + Δh | 6,371,000 m + 818 m | |

r | = | 0.999872 |

r + Δh |

The engineers who designed the Burj would have an error in the fourth decimal place of their calculations. This deviation is probably smaller than the uncertainty in the mass of the girders used to construct this building, which is why Δ*U _{g}* =

*mg*Δ

*h*is totally acceptable for most down-to-earth applications

Now let’s try something astronomical. Can Δ*U _{g}* =

*mg*Δ

*h*be used to measure the gravitational potential energy of the moon? The earth–moon distance (384,400,000 m) is measured from the center of the Earth, not it’s surface. In this case,

*r*+ Δ

*h*will actually be a difference in two numbers.

r | = | 6,371,000 m |

r + Δh | 384,400,000 m − 6,371,000 m | |

r | = | 0.016853 |

r + Δh |

This number is obviously closer to zero than to one, which is why…

U = − _{g} | Gm_{1}m_{2} |

r |

is used for astronomical applications.

### escape velocity

What goes up, must come down. Right? Umm. No. Not necessarily.

Very rough description. Ideas knocked out as quickly as possible.

Using conservation of energy derive the formula for the initial speed needed to escape from a point near a celestial body to a point infinitely far away with a final speed of zero. Write an expression for the total energy (kinetic and gravitational potential) of a moving object initially near a celestial body and then again at some other location.

*U*_{0} + *K*_{0} = *U* + *K*

What value would this expression have if the object was subsequently traveling very slowly and located extremely far away; that is, after it had escaped the celestial body?

− | Gm_{1}m_{2} | + | 1 | mv^{2} = 0 + 0 |

r | 2 |

v = √ | 2Gm |

r |

This is the formula for escape velocity.

### event horizon

A black hole is a star that has collapsed down to a point. Within a certain radius, known as the event horizon, the escape velocity is greater than the speed of light. Since nothing can exceed the speed of light, anything crossing over the event horizon becomes trapped forever within a black hole.

Black holes destroy volume, but not mass, energy, angular momentum, charge, entropy, etc.

Event horizon a.k.a. Schwarzschild radius. The point of no return.

First theorized by John Michel in 1784

Starting from the escape velocity formula, derive an equation for the radius of the event horizon in terms of *m* (the mass of the black hole), *g* (the gravitational constant), and *c* (the speed of light).

v = c = √ | 2Gm |

r_{s} |

c^{2} = | 2Gm |

r_{s} |

r = _{s} | 2Gm |

c^{2} |

Paraphrase this, “it might be possible to make a black hole in a laboratory, nothing lighter than 10 μg can make a black hole, this energy is still out of the range of current particle colliders, but not very energetic cosmic rays, since the Earth hasn’t been gobbled up by baby black holes, we can assume they are unstable.”

### cosmic expansion

When we look at galaxies and other objects outside our own Milky Way we see that they are generally moving away from us and that their recessional velocities us are very nearly directly proportional to their distance. That is, the farther away a particular galaxy is from us, the faster it’s running away from us. If one galaxy is twice as far away from the Milky Way as another, it’s speed will also be twice a great. Three times farther way means three times faster, and so on. This observation was first made in 1929 by the American astronomer Edwin Hubble (1889–1953) and it has since become known as Hubble’s law. Mathematically, Hubble’s law is written…

*v* = *Hr*

where…

v = | recessional velocity (the component of the body’s velocity away from the Milky Way), |

r = | distance from the Milky Way, and |

H = | constant of proportionality known as the Hubble constant.This constant is assumed to vary with time. When referring to its current value we use H_{0}. |

Hubble’s law is important because it tells us that the universe is expanding and, if we extrapolate backward in time, that the universe was initially infinitely small and infinitely dense. It is one of many pieces of evidence for the big bang theory. Space-time came into being some 13.8 billion years ago, was filled with all the mass-energy that exists now and will ever exist, and then inflated rapidly from a region far smaller than a proton to one about the size of a grapefruit in an unbelievably small time of 10^{−32} seconds. This initial out rush of space-time filled with mass-energy was carried by its own momentum until the universe grew to the dimensions we currently see — roughly 13.8 billion light years in all directions.

The universe began with a big bang and is still growing, albeit at a much slower rate than in its early life. The inflationary period when space grew exponentially was quite short. The vast majority of the universe’s history has been one of gradually weakening deceleration. Gravity has been tugging relentlessly on all the mass of the universe ever since its creation, which has slowed the expansion to barely perceptible levels. The increase in the overall size of the universe has meant that the pull of gravity on the cosmic scale has been growing weaker and weaker. The next logical question is, what is the ultimate fate of the universe? Will the expansion carry the galaxies so far apart that they no longer exert any significant gravitational pull on one another or will gravity win out and reverse the expansion of the universe? Will the universe expand forever or will it eventually stop and come crashing back in on itself in a cosmic reversal of the big bang that many call the big crunch? The answer to this question can be found by combining Hubble’s law with the formula for escape velocity. A universe that expands forever will have a density that produces an escape velocity less than that of its observed expansion. A universe that comes crashing back in upon itself will have an escape velocity greater than this. Before we can answer this question we need to begin with a brief explanation of the Hubble constant.

Distances in astronomy are so vast that the meter, or any multiple of the meter, is so hilariously small as to be useless. To get around this, astronomers use one of two units: the light year and the parsec. Of the two, the more intuitive to me is the light year, which is the distance a beam of light would travel in one year (9.46 x10^{15} m). The other unit, the parsec, is a geometric rather than physical unit. Astronomical bodies near to us (well, near in astronomical terms) will appear to shift their position in the sky as the Earth moves around in its orbit. The word parsec stands for “parallax of one arc second”. Thus, an object that appears to shift by one arc second (one 3600th of one degree!) as the Earth moves from one side of the sun to the other six months later is said to be one parsec away. For observational astronomers of the Nineteenth and early Twentieth Centuries, the parsec was a more convenient unit for professional use than the light year. One parsec [pc] is approximately 3.26 light years or 3.09 x10^{16} m. The nearest stars to the Earth other than the sun are a bit more than one parsec away. The edge of the Milky Way is several thousand parsecs away — several kiloparsecs [kpc]. Cosmic distances, like those between galaxies, are measured in millions of parsecs or megaparsecs [Mpc]. This is the unit that Hubble used in his work.

In Hubble’s original paper on this topic published in 1929 he reported a value for his constant of approximately 500 km/s/Mpc. I have included his original data in the section of this book entitled Curve Fitting. You can analyze the data yourself if you wish. Using a standard linear regression analysis, I came up with a value of *H* = 463 km/s/Mpc. The interesting thing about this value is that it is now universally recognized as terribly wrong. Distance measurements of extra galactic bodies in Hubble’s era were later found to be seriously flawed. Still, the theory turned out to be right even if the data used to derive it were all undervalued. Determination of *H* has progressed slowly but surely since the early Twentieth Century and the constant has gone through several revisions. The current most accurate value comes from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP). As of 2014, the WMAP scientists have come up with a value of *H*_{0} = 69.3 ± 0.8 km/s/Mpc. Let’s convert this value to SI units for some perspective…

H_{0} = | 69.3 ± 0.8 km/s/Mpc |

3.08568 × 10^{19} km/Mpc |

H_{0} = | 2.25 × 10^{−18} | 1 |

s |

Seeing the Hubble constant in inverse second form makes it a bit more accessible. The space around us is expanding at a rate of roughly one part in 10^{18} every second. Given that the diameter of a proton or neutron is roughly 10^{−15} m, and that 18 orders of magnitude greater than this 1000 meters, a good phrase to tell your family, friends, and neighbors is that one kilometer of space expands at a rate equivalent to the diameter of one proton every second. If you’d like, we could scale this up a bit timewise.

H_{0} = | 2.25 × 10^{−18} | | 10×365.25×24×3,600 s |

s | decade |

H_{0} = | 7.09 × 10^{−10} | 1 |

decade |

This is a bit bigger than the diameter of a typical atom. Thus, one meter of space expands at a rate equivalent to the diameter of an atom every decade. An expansion this slow is imperceptible on the scales we humans are used to. Our lifetimes are too short and the sizes we have to deal with on a daily basis are too small.

To begin to appreciate the Hubble constant we need to scale things up a bit sizewise — a vast bit — 23 orders of magnitude and beyond. We need to look at the universe as a collection of galaxies; the closest of which are a few million light years away (10^{23} m) and the farthest of which are ten billion light years away (10^{26} m). The current most distant known objects are quasars. A quasar is a galaxy with a super massive black hole at its core that is actively gobbling up stars. The most distant quasar is moving away from us at 90% of the speed of light. Using Hubble’s law, this gives us a distance of 12.4 billion light years.

r = | v | = | 0.90 c | |

H_{0} | 2.25 × 10^{−18} s^{−1} | | ||

r = | 4.01 × 10^{17} light seconds | | ||

| ||||

r = | 4.01 × 10^{17} light seconds | | ||

365.25 × 24 × 3,600 s | ||||

r = | 12.7 × 10^{9} light years | |||

Now let’s use Hubble’s law to answer my original question, will the universe expand forever or will it stop like a ball tossed in the air and come crashing back in on itself? We’ll calculate the critical density, the value that separates these two outcomes, by setting the escape velocity formula equal to the velocity from Hubble’s law. Square both sides to eliminate the square root.

v = H_{0}r = √ | 2Gm | ⇒ | H_{0}^{2}r^{2} = | 2Gm |

r | r |

Here comes the tricky part. What’s the mass of the universe? Determining this number is quite difficult, but by looking at a large portion of the universe we can determine its density. Density times volumes gives us mass. Looking out from our vantage point in the Milky Way, we can see equally well in nearly all directions. (The Milky Way is rather congested, which prevents us from seeing those portions of the universe lying along its plane, but this doesn’t hurt things much.) The locus of all points equidistant from a fixed point generates a sphere in space. Multiplying the density of the observable universe times the volume of the sphere that contains it gives us its mass.

m = ρV = ρ | 4 | πr^{3} |

3 |

Substitute this expression into the one derived above it and solve for density. This is the critical density separating eternal expansion from eventual collapse.

H_{0}^{2}r^{2} = | 2G | | 4πρr^{3} | ⇒ | ρ_{0} = | 3H_{0}^{2} |

r | 3 | 8πG |

Let’s compute its value using the current best estimate of the Hubble constant…

ρ_{0} = | 3H_{0}^{2} | = | 3(2.25 × 10^{−18} s^{−1})^{2} | |

8πG | 8π(6.67 × 10^{−11} Nm^{2}/kg^{2}) | | ||

ρ_{0} = | 9.02 × 10^{−27} kg/m^{3} | |||

Anything greater than this density will resist the expansion of space. You and I and everything we see in our ordinary lives has a density much greater than this and will retain its shape and size for as long as all other things remain constant. People and other animals have a density about the same as water — 10^{3} kg/m^{3}.

Looking at the universe as a whole, however, the situation is somewhat different. Given that most of the universe is hydrogen and that the mass of one hydrogen atom is 1.67 x10^{−27} kg, this corresponds to a density of a half dozen hydrogen atoms per cubic meter.

ρ_{0} = | 9.02 × 10^{−27} kg/m^{3} |

1.67 × 10^{−27} kg/H atom |

ρ_{0} = | 5.40 | H atom |

m^{3} |

Our galaxy, and probably every other galaxy in sight, has a density of roughly one hydrogen atom per cubic centimeter. Since there are a million cubic centimeters in a cubic meter, galaxies are more than dense enough to resist the expansion of space. About the only thing that can’t is the space between galaxies, where the density is apparently on the order of one hydrogen atom for every four cubic meters.

The Hubble constant appears *not* to be constant. Recent observations from the Wilkinson Microwave Anisotropy Probe — the same spacecraft responsible for the current best value of the Hubble constant — show that the rate of expansion of the universe is increasing. If the universe began with a big bang 13.8 billion years ago it just might end with a big rip, 20 billion years in the future. The reason is that there’s more to the universe than meets the eye. Much more.

What you see isn’t what you get.

Only 4% of the universe is thought to be made of ordinary matter — mostly atoms, nuclei, electrons, photons, and neutrinos. Another 23% is dark matter — “dark” because it interacts gravitationally like ordinary matter, but not electromagnetically. Thus it cannot be “seen” with electromagnetic radiation like light, x-rays, or radio waves. (Dark matter is discussed in more detail in a previous section of this book.) The remaining 73% of the universe — the overwhelming majority — is in the form of dark energy. Dark energy is really odd, not only because it can’t be seen, but also because it acts like negative mass. That is, it strives to expand space rather than compress it. Despite its preeminence, dark energy has only the feeblest effect on the local space around us. (In this context “local” refers to everything within out galaxy and nearly everything from here to the nearest thousand galaxies.) Small though it is, this tiny effect adds up until it dominates the behavior of the space in the universe as a whole. Dark energy may be tiny but it is everywhere. Like a swarm of mosquitoes at a summer picnic, dark energy has enough of an effect to ruin the whole party.

With the way things are going, in another ten billions years or so the distance between galaxies will have expanded to the point where light won’t be able to traverse the space between them fast enough. Space will be expanding so rapidly that every galaxy will be independent of and invisible from every other galaxy. Our visible universe would then be reduced from its current 100,000 galaxies to just one — our own Milky Way.

Now it gets really weird. If there’s nothing to stop the acceleration of the expansion, eventually the galaxies themselves will start to come undone. (This is the so called phantom energyscenario.) First, the 100 billion stars of the Milky Way will split off to form isolated solar system universes. Then the space within these solar systems will begin to increase noticeably. The earth and other planets will recede away from each other in our own solar system. The sun would grow ever smaller in the sky until it disappeared. At this point, life on earth would get really bad. (Assuming the Earth was still around.) Worse than the thought of isolation from the rest of the universe is the thought of losing touch with the sun. This would be our effective end. Were anyone around to witness what happened next, they would eventually see the rate of expansion increase to the point where the Earth exploded, then everything on the Earth would be reduced to vapor, then molecules would lose coherence, followed by atoms and nuclei. The final end of it all would occur when the fundamental particles themselves were torn to shreds and there was nothing left of the universe to say that it ever existed.

I’ll tell you what I like about this stuff. It forces you to think beyond yourself. What will the world look like when you die? The answer is culturally quite different (it’s the nature of times we live in) but physically pretty much the same. Multiply this by ten, by a hundred, by a thousand, by a million lifetimes! Care to make a prediction? Keep going. A billion. A trillion lifetimes. Now the time scales have gotten so vast that only a few would care to speculate. This is the realm of theoretical physics and, amazingly, it is *not* beyond our comprehension.