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Hi I am a high school physics student and I am currently struggling to grasp the concept of Newton's law of universal gravitation.

I am really confused with how this works. So do all objects attract each other with a force of gravitational attraction?

So when I am looking at my laptop which is stationary on a table, there is force of gravitational attraction between me and the laptop?

If so, how come I am not attracted to the laptop, like how come I am not moving towards it?

So does Newton’s law of gravitation apply to any and every object in the universe, no matter the size and the mass?

Qmechanic
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CountDOOKU
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2 Answers2

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So does all objects attract each other with a force of gravitational attraction

So when I am looking at my laptop which is stationary on a table, there is force of gravitational attraction between me and the laptop?

So does the newtons law of gravitation apply to any and every object in the universe, no matter the size and the mass?

In the context of high-school physics: Yes to all of these.

Of course, later you will learn that Newtonian gravity is an approximation to General Relativity, which is itself expected to be an approximation to an undiscovered theory of quantum gravity. But to the best of our knowledge all objects interact gravitationally, regardless of their size.

If so how come i am not attracted to the laptop, like how come am i moving towards it?

You are. It is instructive to approximate the magnitude of the force. Suppose that you mass 70 kg and the laptop masses 1 kg. Approximate both as point masses located 50 cm apart. The gravitational force is then about 19 nN. This is far less force than is exerted by the laptop’s fan exhaust, maybe not even enough to move a grain of dust against friction.

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Dale
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  • I think the first part of the answer should be "all massive objects interact gravitationally"? – Allure Feb 25 '20 at 03:28
  • @Allure Massless objects also interact gravitationally. In fact, radiation is an important source of gravity in the Friedmann equation used in the current cosmology model, and some pp-wave spacetimes also describe the gravitation of massless radiation. So massless objects not only experience gravity but they are also valid gravitational sources. – Dale Feb 25 '20 at 16:34
  • @Dale would the distance be the distance between the centres of the 2 objects or the surface of 2 objects? Is that what "point masses" refer to? centre of 2 objects? Sorry i am new to the terminology. Many thanks! – CountDOOKU Feb 26 '20 at 02:59
  • @FredWeasley Yes, the easiest approximation is to consider the two objects as though all of their mass were concentrated at the center of mass of each object. If you don't make that simplifying assumption then you have to integrate the force law over the object. It can be done, but the math gets messy fast, and it will be a small correction to an already small effect. – Dale Feb 26 '20 at 15:45
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It might also help to understand that gravity is the weakest of the 4 known elementary forces. It is way weaker than the electro-chemical forces keeping your shoes from falling apart & sliding on the floor, and the feet of your computer on the table, that cause friction, which resists the motion that would very slowly cause you two to float together. Even the reaction jets of your breath flowing out of your nostrils and the computer cooling fan, are millions of times greater than your gravitational attraction. That said, if you stand a yard from your computer which is on a podium, and you are both in the middle of a salt 'flat' you will 'lean' away from your computer slightly (to feel 'vertical') because of your mutual gravity, but mostly because of the curvature of the Earth over that 3 foot gap. It takes trillions of times your mass (the Earth) to generate the force needed for your mass to feel the weight you have.

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