Astronomy Common Knowledge Quiz.......
Quizzes & Puzzles1 min ago
Gravity
Can anyone explain to me (in laymens terms) exactly what gravity is?
I dont want links to websites explaining it as Ive looked at those and cant make head nor tail of them. I just want a simple explanation why things fall down and not up.
Why does the earth keep the moon in orbit? Apparently the more matter the more gravity right? So why cant I orbit a frozen pea around a football? Come to that, why hasnt the moon fallen to earth yet?
Dont expect I'll wrap my head around it but humour me anyway.
I dont want links to websites explaining it as Ive looked at those and cant make head nor tail of them. I just want a simple explanation why things fall down and not up.
Why does the earth keep the moon in orbit? Apparently the more matter the more gravity right? So why cant I orbit a frozen pea around a football? Come to that, why hasnt the moon fallen to earth yet?
Dont expect I'll wrap my head around it but humour me anyway.
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jim360
//Remarkably, the Moon is actually slipping out of Earth's grasp, rather than falling in. Again, this is associated partly with how weak Gravity is. The effective force that's making the Moon want to leave (this is centrifugal force, due to the Moon's orbit) is stronger than Gravity that's keeping it in. //
No. The Moon is receding from the Earth because it is losing orbital momentum which is being transferred to the rotational momentum of the Earth through the friction encountered by the tides.
//Remarkably, the Moon is actually slipping out of Earth's grasp, rather than falling in. Again, this is associated partly with how weak Gravity is. The effective force that's making the Moon want to leave (this is centrifugal force, due to the Moon's orbit) is stronger than Gravity that's keeping it in. //
No. The Moon is receding from the Earth because it is losing orbital momentum which is being transferred to the rotational momentum of the Earth through the friction encountered by the tides.
I didn't research so I'll take your word for it beso. Probably you're right -- although the point is that gravity is indeed not strong enough to hold on to the Moon indefinitely, and other effects dominate.
divebuddy, the confusion is because all of those objects are very massive, but then you need one heck of a lot of mass for gravity to be particularly strong. If you want to see how weak it actually is, you need to compare it against other forces on the same sort of scale.
Consider, for example, an experiment in which two large iron balls weighing about 15 kilograms are placed a metre apart from each other, and then given a total charge of 15 Coulombs (which is the electricity equivalent of a kilogram, and I've chosen a number that is about the amount of charge in a typical lightning bolt). Plug in the numbers to the respective formulas and you find that the electric force between the two balls is about 100 million million million times stronger.
Gravity is only strong, then, on stupendously large scales. This is partly why it took so long for General Relativity to come around: it took us that long to be able to notice it.
divebuddy, the confusion is because all of those objects are very massive, but then you need one heck of a lot of mass for gravity to be particularly strong. If you want to see how weak it actually is, you need to compare it against other forces on the same sort of scale.
Consider, for example, an experiment in which two large iron balls weighing about 15 kilograms are placed a metre apart from each other, and then given a total charge of 15 Coulombs (which is the electricity equivalent of a kilogram, and I've chosen a number that is about the amount of charge in a typical lightning bolt). Plug in the numbers to the respective formulas and you find that the electric force between the two balls is about 100 million million million times stronger.
Gravity is only strong, then, on stupendously large scales. This is partly why it took so long for General Relativity to come around: it took us that long to be able to notice it.
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@beso
//It doesn't exist in an inertial frame of reference. However it can if you adopt the frame of reference of the orbiting object.//
The way centripetal force was taught to me involves the constant change of direction of the object's velocity vector, which is treated as an acceleration.
Even those who hold by centrifugal force instinctively know when to let go of a sling, or release the discus and that a car hitting ice in mid-corner goes into straight-line motion not a sudden perpendicular change of direction.
Continued motion in a straight line, unless (etc) should have a familiar ring to many.
//It doesn't exist in an inertial frame of reference. However it can if you adopt the frame of reference of the orbiting object.//
The way centripetal force was taught to me involves the constant change of direction of the object's velocity vector, which is treated as an acceleration.
Even those who hold by centrifugal force instinctively know when to let go of a sling, or release the discus and that a car hitting ice in mid-corner goes into straight-line motion not a sudden perpendicular change of direction.
Continued motion in a straight line, unless (etc) should have a familiar ring to many.
