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

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Gauss law in gravitation

Is it possible to use Gauss's law of electromagnetism, (The net electric flux through any closed surface is equal to $1⁄\epsilon$ times the net electric charge enclosed within that surface.) to ...
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1answer
166 views

Would a non-rotating Earth collapse on the Sun?

If the earth stopped rotating on its axis, would this influence its revolution motion? In particular, could it collapse on the sun? I ask this because on the one hand I thought the two degrees of ...
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1answer
45 views

Will the Earth ever show synchronous rotation? Why and when?

In this answer to the question "why does the moon have the same rotation and revolution periods?", we read: The mass and speed of rotation of the Earth influence the moon in that some of its ...
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3answers
142 views

Newton's gravity vs Einstein's gravity

Hello so if i were to simplify the difference between gravity as seen from Newton and gravity as seen from Einstein, could i say that Einstein's gravity is just adding the fact that gravity does not ...
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1answer
42 views

Conservation of energy in the “ideal physics world”

Let us assume we are in the textbook ideal physics world, without any friction. Now, if I place a ball at the top of a parabola U and let it go under gravity, the ball should roll up and down forever ...
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2answers
101 views

How to find speed of rotation of gears from kinetic energy of falling body?

A pulley is fitted to the first driven shaft of compund gear train. Last driven shaft of gear train(gear 8) is fitted to the 12V DC generator (6W).Now to the rope extension at the one end of the ...
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0answers
35 views

Pendulum in radial gravity field

All I could find about pendulums assumes that the force on the pendulum mass m is mg directed downwards. The case of m being attracted only by the radial gravity pull (thus replacing the "plane" ...
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2answers
65 views

Planets potential [duplicate]

I'm a physic student of Italy. I'm moving my first steps in this beautiful world, and as every new entry, my simple and stupid questions are sometimes not so simply resolved. I'm trying to know the ...
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0answers
40 views

Semi-major axis , specific mechanical energy, its period and velocity at Apogee and Perigee and what the difference is [closed]

Hi im needing some help with two questions could you please show the formula and the way its solved?Thanks PS:Could you also explain what the difference of velocity and apogee and perigee? 1.)A ...
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2answers
119 views

In what direction should you throw a 1Kg uniform sphere in order to put it into lower earth orbit? [closed]

If you are standing on earth's surface, in what direction (and at what speed) should you throw a 1Kg uniform sphere of radius 0.1 meters in order to put it into lower earth orbit? Assume that there ...
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2answers
75 views

Why do we take $h$ as “height from surface to bottom” when calculating liquid pressure?

In the following image, pressures of points x, y and z are $P_{x}, P_{y}$ and $P_{z}$ respectively, and they all are equal. My question is, why? The amount of matter on x is much more than amount of ...
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1answer
37 views

Motion under gravitational forces

In a system of particles, can a particle be considered to be attracted to the combined center of mass of the other particles if the Newton's universal law of gravitation is used (the inverse square ...
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1answer
535 views

What is the difference between gravitational force and gravitational field?

I see two different formulas describing gravity: $$F=\frac{GMm}{r^{2}}$$ $$g=\frac{F}{m}$$ But I don't understand the difference between gravity as a force and its field as a vector.
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2answers
47 views

Parachute Model

Can you tell me if this model is correct or not? [assuming drag is linearly proportional to the velocity] Considering y-axis vertical and x-axis horizontal,with y positive upwards: 1) $F_y=-mg-kv_y=...
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1answer
54 views

Force felt by parachutist when opening the parachute [closed]

When falling down with air resistance, y-axis vertical (positive is up), x-axis is horizontal, we have that: $F_y=-mg-kv=ma_y$, where $k$ is the drag coefficient. Now if the man opens the parachute, ...
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1answer
52 views

Hot air balloon trajectory predictions

Hey there Physics friends! I've been struggling with what seems to be a very basic physics question. Let's say that I have wind vector for a given altitude and location, and I want to calculate the ...
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5answers
97 views

How can escape velocity and gravitational potential be higher in weaker gravitational fields?

on Earth we have an escape velocity of about 11180 m/s and a gravitational field of g=9.81 m/s^2. It would seem obvious that the escape velocity was higher for stronger gravitational fields. Often it ...
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2answers
57 views

Free fall vs throw with air resistance

If I let a stone drop from a certain height and throw a stone from a certain height, with air resistance, which will reach the ground first? When I draw a free body diagram the air resistance in the ...
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3answers
78 views

Could the Earth-Moon rotation period equal one Earth year?

I just stumbled on this rather confused QA. The point seems to be if the Earth-Moon rotation period was identical to the Earth-year, then ... ... all three would stay in a line at all times. ...
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5answers
91 views

Solving for $r=0$ in an inverse square law

This occurred to me in thinking about Coulomb's Law for a spherical insulator, but obviously can be asked of any inverse square law force calculation. For example, all things on the surface of he ...
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2answers
62 views

The shortest path among two points inside Earth [closed]

I have this idea and I don't know how to process, explain or question it. I hope you can understand these images and help me formulating a good question. This is like a gravitational train but it ...
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4answers
95 views

Gravity and force

I have a question and am not able to answer it. Suppose you drop two objects from different heights. They are exactly the same shape, size and weight. Now we know that the gravitational acceleration ...
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1answer
100 views

Distance from Earth where gravity becomes negligible

This is a basic little problem I thought up when trying to remember some physics, and I wanted to see if it's at all correct: trying to figure out the distance from earth where gravity becomes ...
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0answers
21 views

Gravity changes at the equator due to Earth's spin [duplicate]

If an object weights 100 kilos at the equator how much would it weight if the earth spun twice as fast?
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6answers
6k views

How does a spinning object “know” that it is spinning?

I am constructing a thought experiment about a spinning object that is floating in intergalactic space. I assume that this object is about the size of a planet so that it will have enough gravity so ...
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2answers
51 views

Where is the space elevator falling? And the cable holding it?

I guess the concept of a space elevator is pretty well known. The idea, first published by Konstantin Tsiolkovsky in 1895, and popularized (among others) by Arthur C. Clarke in The Fountains of ...
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0answers
32 views

Blackhole and light [duplicate]

I was talking with my friend about blackholes and I asked him "Why blackhole 'sucks' the photons?" He had no deeper understanding so he answered that "I dont know, may the light is affected, because ...
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1answer
46 views

Forces in a body falling with air resistance

I would like to know why when a body falls with air resistance, we have that the drag caused by the air is -kv and the acceleration is -mg. What I don't understand is how they both have the same sign ...
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1answer
74 views

Relation between gravitational field and gravitational potential

The gravitational field is the negative differential of the gravitational potential. Now the gravitational potential due to a particle at a distance $r$ is $-Gm/r$ where $m$ is the mass of the ...
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3answers
133 views

Using normal rules of gravity, How would gravity work on a flat “world”?

The question is a bit vague, so let me provide some more context. Imagine we have a world like Middle-earth (prior to the end of the Second Age), or any of many fictional Fantasy worlds (Glorantha, ...
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27 views

How to calculate the force of pull when a high speed rotating wheel is braked suddenly and immediately accord to angular momentum?

According to angular/circular momentum, how can I calculate the force of pull when a high speed turning wheel is stopped suddenly and immediately (I mean stop to 0 m/s within 0.1 sec). As I know, this ...
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2answers
113 views

When a rocket or car is accelerating is the air in it going backwards or not?

I'm not sure but when a car is accelerating is the air in the car pushed backwards or not? If so why did Einstein say that there is no way of finding out (besides tidal effects) the difference ...
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40 views

Centripetal Force Solving

I was recently learning about centripetal force when I came across a problem, I felt with my calculations. I re-watched the video, and the answer seems inadequate. The problem involves a bicyclist ...
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2answers
92 views

Why is it called “escape velocity” and not “escape acceleration”?

As we know, velocity to escape from an orbit is in proportional with the orbital velocity: $$v_\mathrm{escape}=\sqrt{2}v_\mathrm{orbit}$$ Since, orbital velocity decreases as we move away so should ...
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2answers
59 views

What is the connection between Newton's laws of motion and his law of gravitation?

What is the connection between Newton's laws of motion and his law of gravitation? Is there a connection where one suggests the other, or are they just two separate ideas that are assumed to be true?
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62 views

Ignoring orders of magnitude in a problem

I am so lost in this solution to a physics problem. In the problem I find the gravitational force on the Moon due to Earth and the gravitational force on the Moon due to the Sun They are about $1....
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1answer
73 views

Universal Law of Gravitation

Suppose that there are two bodies A and B in a vacuum with equal rest mass M and at a distance D apart. The universal law of gravitation states that, if each is at rest relative to the other, the ...
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1answer
97 views

Speed of satellite when it falls freely on the surface of the earth [closed]

I am a high school student in my final two years before university. I have a question which I do not know how to approach. Any help would be appreciated. Thanks! "If an artificial satellite is ...
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1answer
49 views

Gravity and Newton's 2nd law

I started to wonder what would be different when opening e.g. a very heavy door (e.g.m=10 tons) of a safe here on Earth and on the Moon. Lifting (!) the door (taken of its hinges) straight up here on ...
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1answer
79 views

Can the value of $c$ be calculated from Relativity? [duplicate]

I have three related questions about what theories or equations can calculate and what is an input parameter/constant to the theories. This question is not about parameter units (as another similar ...
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1answer
53 views

Solidifying understanding of centrifugal force at the equator vs poles

I'm just trying to get my head around the physics here and understand exactly what's going on, because there's a lot of conflicting information on the internet. Viewed from an inertial reference ...
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1answer
48 views

Calculating orbits with varying laws of physics? [closed]

The general way planets orbit stars in our universe is determined by the inverse square law and the three laws of motion. Both the inverse square law and the three laws of motion are the reason that ...
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2answers
57 views

Momentum when a people fall down to ground (Related to momentum, gravity, lever)

According to the Newton's laws, free fall is subjected to the pull of gravity at acceleration of 9.8 m/s per second. (Something like this) But I don't think the principle of "free fall" is applicable ...
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1answer
60 views

Why can't we say that moon attracts the earth?

We know that the Gravitational force at which earth attracts moon is also equal to the force in which moon attracts earth as the distance and the G and the masses are same then why can't we say that ...
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2answers
184 views

Can Newton's law of gravitation and Einstein's GR be considered true and complete laws of gravitation? [closed]

Newton's law of gravitation and Einstein's general relativity (GR) are empirical laws of gravitation that are strong on description and prediction but lack deterministic and/or causal rigor, which ...
2
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1answer
32 views

Speed of pinewood derby car

If I drop a pinewood derby car from 4 feet I know it hits the ground in half a second at about 10.6 mph. If it rolls down the track (starting at 4 feet) it takes longer, but is it possible for it to ...
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6answers
168 views

People went down two different length slides end up at the bottom at the same time [closed]

Discussion I am having: If there are two slides that are at the same height. One slide is 100m long and the other slide is 200m long. The endpoint and start point are the same displacement. In ...
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6answers
255 views

If gravity is additive, then how does it cancel itself out?

I understand that gravity, as far as we know, is always attractive. Also, it has additive qualities - i.e. the size and strength of the field are proportional to the quantities of mass. This seems to ...
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2answers
160 views

Why do stellar boundaries exist?

What theorems or insights are available to decide wether a star with some given interdependence between its density, pressure and temperature distributions ought to have a boundary at a finite ...
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1answer
76 views

Gravitational force between rod and mass $m$

Why do we integrate along the whole length while finding the gravitational force between a object of mass ($m$) and a rod of length $L$ and mass $M\,?$ Can't we simply use $F=\frac{GmM}{r^2} \, ?$