The Newtonian model of gravity in which the force between two objects is given by GMm/r^2.

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22
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2answers
589 views

Hourglass on the Moon

Someone asked me this, and I was surprised to find I couldn't answer it: suppose I have an hourglass / egg timer that times two minutes in Earth's gravity. If I used it on the Moon, how long would it ...
38
votes
8answers
6k views

“Falling upward” - how far you have to be from Earth to start falling to the Moon?

Talking about gravity with my 9 y/o she asked when do we start "falling upward" to the Moon. What is the distance at which the Moon's gravitational attraction is higher than that of the Earth and thus ...
2
votes
2answers
138 views

Could one make steel balloons? [duplicate]

Reading this question reminded me of an idea I once had, and I wonder whether it's feasible. Imagine a hollow ball made of something strong, like steel. Imagine there is nothing inside; only vacuum. ...
0
votes
2answers
242 views

How can Rosetta spacecraft orbit an object with such a low mass?

Comet 67P Churyumov–Gerasimenko's mass is 11 orders of magnitude lower than Earth's. That means that the comet's gravity force is also weaker than Earth's in the same proportion. Provided that also ...
0
votes
4answers
441 views

Car driving / falling off of a cliff - will it land upright?

In a recent episode of The Walking Dead, a van was pushed off of a bridge and flipped once and landed upright, on all four wheels. Assumption: the ground and cliff are both flat. Will a vehicle ...
2
votes
1answer
201 views

Rate of acceleration for two hanging masses in two experiments

I conducted an experiment where two masses of different weight were attached by a string and fed through a pulley. The difference between the masses was $0.025\: \mathrm{kg}$. The lighter mass was ...
0
votes
1answer
88 views

Newton's gravity formulas for ellipses

A few times I've come across formulas, which are identical for circular and elliptical orbits, except that for the second ones you substitute $r$ with $a$, where $r$ is the radius of a circular orbit ...
0
votes
1answer
171 views

Direction of gravitational field in Hemisphere

Supposing we have a uniformly mass distributed hemisphere of some radius, so what will be direction of gravitational field throughout the flat surface of hemisphere? And why?
2
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2answers
1k views

What is the weight of the Philae lander on the Churyumov–Gerasimenko comet compared to earth?

We know the payload mass of the Philae lander was 21kg. We know the mass of the Churyumov–Gerasimenko comet is roughly 1 x 10^13kg. We know the mass of Earth is roughly 5.9x10^24kg. I've heard ...
3
votes
2answers
150 views

How is the most accurate value of $G$ measured?

How do modern scientists update the measurement of $G$, the gravitational constant? Is CODATA the authority on this measurement and the experiment?
0
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7answers
477 views

Could Galileo ever prove that $g$ is the same for a feather and a hammer?

Do you think he could have possibly found a way to make a feather and a hammer touch ground simultaneously, as they should, according to gravity? Can you make up a device that avoids air resistance? ...
3
votes
1answer
292 views

How would the Sun's gravity affect us if we weren't in orbit?

So since we're in orbit around the sun, we are basically in free-fall. From what I understand, this means that we don't feel the gravity of the Sun like we do the Earth. Just like astronauts on the ...
0
votes
4answers
173 views

Solar vs lunar gravity: inverse square law

Bearing in mind the moon's perfect fit when eclipsing the sun, were the two orbs to have the same density, would they not each exert the same gravitational pull on the Earth? (Much like a dim star in ...
0
votes
3answers
230 views

Angular momentum

The satellites are revolving around a planet in circular orbits by themselves (no external source to maintain their speed). Are we able to make them revolve in elliptical orbits by themselves? Do ...
4
votes
1answer
505 views

Philae lander simulation off by factor of ~3

I'm trying to simulate the Philae landing by writing a program to compute the position of the lander vs time. According to various mission websites, the orbiter will match its orbit to the rotation of ...
23
votes
5answers
7k views

Build a ring around Earth, then remove the supports

What would happen if we decided to build a giant ring that managed to wrap around the whole world, end to end that was supported with pillars all along the ring and then the supports all suddenly ...
0
votes
1answer
47 views

Why do we have to use an integral in this scenario to figure out $v_{max}$? [duplicate]

$$\int_0^R mg\frac{r}{2}dr=\frac{m}{2}v_{max}$$ and can't write $$\frac{m}{2}v_{max}=mgR$$ don't we usually equal potential energy to kinetic energy? A stone is falling through a tunnel towards the ...
0
votes
1answer
88 views

Is there anything interesting about an n-body system whose potential energy is constant?

I am curious about a system of gravitating bodies with particular characteristics. Before describing the characteristics of the system, let me say what I want to know about it: Can this be a ...
4
votes
2answers
1k views

Platform diving: How deep does one go into the water?

I am a platform diver. I am trying to figure out how deep in the water I go depending on which platform I dive from. The platforms are 1m, 3m, 5m, 7.5m and 10m above the water surface. By ignoring ...
2
votes
2answers
4k views

Why does the acceleration $g$ due to gravity not affect the period of a vertically mounted spring?

For a vertically mounted spring, I was looking at the formula $ T= 2\pi \sqrt{m/k}$ for a period. Why doesn't the gravitational acceleration $g$ factor in?
0
votes
2answers
57 views

Human between two bodies of large mass

What if a person is put between two identical dense spherical bodies (well, not as dense as black holes, but definitely 100 times more than earth) with mass $M_1=M_2=10^{30}kg$, exactly in the middle ...
0
votes
2answers
209 views

Correlation between equations of elliptical orbits and pendulums

The equation for the period of a pendulum is: $$T=2π\sqrt{\frac{L}{g}}$$ Where 'g' is the acceleration due to the gravitational field and 'L' is the length. The equation for the period in of a body ...
8
votes
7answers
905 views

Distribution of Gravitational Force on a *non-rotating* oblate spheroid

Suppose a person is standing on a non-rotating$^1$ oblate spheroid of uniform density. He first stands on one of the poles, then on the equator. In which case is the gravitational force greater? In ...
0
votes
3answers
404 views

How much does the Earth's population affect the force of gravity? [duplicate]

If everyone on earth except one person were to gather up in one location, say the North Pole, and jump at the exact same time and stay airborne for about 1.5 sec. And that one person who did not jump ...
3
votes
2answers
269 views

Kepler's First Law of Planetary Motion: Whats at the other focus? [duplicate]

Kepler's first law states that the orbit of every planet is an ellipse with the Sun at one of the two foci. Is there anything significant about the location, or any unusual properties about the other ...
1
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1answer
60 views

Projectile motion. Is there a point where sum of the forces is 0?

Body of mass m, is thrown with initial speed, at an angle α from a horizon. Neglecting wind resistance, is there a point on the trajectory where the sum of forces is 0? I don't think so, but maybe at ...
0
votes
1answer
107 views

Two-Body problem confusion regarding equations of motion

I'm reading John Taylor's Classical Mechanics text, and I'm a little confused about something in chapter 8, which deals with the two-body problem with central forces. The issue is with the equations ...
0
votes
1answer
228 views

Question regarding calculating acceleration due to gravity on planet Mercury [closed]

I was asked to calculate the acceleration due to gravity on planet Mercury, if the mas of Mercury is $2,99 \times 10^{22} kg $ and its radius is $2,42 \times 10^{3} \ km$. The mass of the object is $...
6
votes
4answers
603 views

Can the gravitational constant $G$ be calculated theoretically?

We know all that the gravitational constant is $$G=6.67428±0.00067\times 10^{-11}\mathrm{m^{3} \:kg^{-1} s^{-2}}.$$ But can we calculate it theoretically?
-1
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1answer
1k views

how can an egg be balanced on a nailhead on equator [closed]

How can an egg be balanced on a nail head on equator? Why only on equator and not on different latitudes?
4
votes
2answers
350 views

What is the cause of planetary rotation. Not orbital rotation [closed]

I am a senior in high school, sixteen years old. I think differently than most in my class. Recently, I have been thinking about alternative causes for the rotation of our planets. My current thought ...
1
vote
2answers
365 views

What is the jerk due to gravity with ascent? [closed]

Acceleration is defined as the rate of change of velocity with time. Jerk is defined as the rate of change of acceleration with time. What is the jerk due to gravity with ascent?
1
vote
2answers
118 views

Does a ball spin more from left to right than it does from right to left?

I swear that in a manual on tennis published by the German Tennis Association (Deutscher Tennis Bund) it stated as fact that all things being equal a ball will move more from left to right than from ...
-1
votes
1answer
253 views

Explain the terminal velocity equation [closed]

As explained by Method 1 on WikiHow, the equation for Terminal Velocity is: $$ V_T = \sqrt{2 \times m \times g \over ρ \times A \times C} $$ Some parts of this equation are self explanatory, and ...
2
votes
3answers
713 views

Can Newton's Law of gravity be deduced using dimensional analysis?

I tried using dimensional analysis to deduce Newton's law of gravity but I wasn't able to do so as one of the equations were $0=-2$ which is a contradiction. But I thought that we can't do that ...
15
votes
5answers
11k views

How fast would someone have to run to travel vertically up a wall?

I am currently doing a physics project on the effects of so-called 'super-speed'. I was wondering how fast you would have to run to vertically travel up a wall? That is, to negate the force of gravity....
3
votes
1answer
91 views

Why does matter gather as discs around massive objects? [duplicate]

Why do discs, like rings of Saturn and the spiral shape of our galaxy form around massive objects, instead of just a (spherical?) cloud of matter?
4
votes
3answers
290 views

Why is $\pi$ used when calculating the value of $g$ in pendulum motion?

I am trying to intuitively understand why $\pi$ is used when calculating the value of $g$ using the harmonic motion of a pendulum: $$g ~=~\frac{4\pi^2L}{T^2}.$$ Does it have something to do with the ...
0
votes
1answer
114 views

What is $g$ when traveling quickly around a planet?

I am trying to model a rocket orbiting a planet. I want to simplify the problem so that the rocket is travelling along an infinitely long flat surface with a $g$ along the $y$-axis. This is what I ...
2
votes
3answers
758 views

While finding escape velocity, does human mass counts as the mass of Earth?

Escape velocity depends upon the mass of earth. my doubt is whether human mass adds into it or not? If it add then Earth's escape velocity would be increasing!
5
votes
6answers
4k views

Why does the moon stay with the Earth?

Naturally, I know it to be true that the moon goes around the Earth and that the Earth goes around the sun. However, attempting to picture this has confused me a bit. Why doesn't the moon just get ...
6
votes
4answers
29k views

Why is Earth's gravity stronger at the poles?

Many sources state that the Earth's gravity is stronger at the poles than the equator for two reasons: The centrifugal "force" cancels out the gravity minimally, more so at the equator than at the ...
10
votes
2answers
1k views

Why heavy elements don't sink to the core?

If earth assembled out of space dust, how come we find heavy elements like gold, silver, uranium and bunch of others that are heavier than iron on the surface? I mean silicon (Si mass 28.084) being ...
0
votes
1answer
158 views

How would the solar system change if the Earth were removed

I was reading a closed question with the same title and wondered if it would be on-topic with the following change: Could a large-mass object from interstellar space pass through the inner solar-...
1
vote
5answers
1k views

Why is kinetic energy stored as potential energy in a body during its motion against gravity?

Everyone knows kinetic energy is converted to potential energy in the body when it moves up against the earth's opposing gravitational force. But I am facing some problem with this. What I learnt ...
0
votes
3answers
9k views

When does acceleration due to gravity equal positive/negative? [closed]

For example a projectile is launched at an angle. What would $a$ in $y=vt +.5at^2$ be? Let's say I choose up to be positive. How do you not confuse yourself whether to use positive or negative $a$?
0
votes
2answers
686 views

Space Elevator: how would you keep the end of a Space Elevator's cable rigid?

I had a discussion with my wife about physics behind a Space Elevator. Imagine a cable rising 60k into the firmament for its elevator to climb. The cable would have to rotate in unison with the earth, ...
0
votes
1answer
36 views

Correcting for oblateness of the Earth [duplicate]

I'm trying to model numerically the effects of the Earth's oblateness on satellite orbits. I'm seeing precession of the orbit, but not in the plane that I'm expecting. Here's my corrected grav. ...
2
votes
3answers
356 views

Two bodies of finite size treated as two point masses in Newtonian gravity

When discussing gravitation between two bodies of finite size, for instance Earth around the Sun, we suppose the mass of Earth and the Sun to be perfectly localized at the center of each body. Is this ...
0
votes
1answer
110 views

Why is $d$ generally not used instead of $r$ in Newton's derivation of force of gravitation? [closed]

In Newton's law of gravitation we take distance between two bodies $r$ and represent it in the form of $r^2$? Why don't we take distance as $d$?