Ok, so this may sound a bit odd but gravity has always confused me somewhat. As I understand it, gravity is the force responsible for pulling objects down to Earth. Therefore, it must be an immensely powerful force. However, a fridge magnet stays on the fridge, a poster blu-tacked to a wall does not fall down etc. My questions are these: How can a force which is capable of pulling large vehicles and incredibly heavy objects to Earth not manage a small magnet or piece of blu-tack or any adhesive?How can a force be so strong yet paradoxically be so weak? How does gravity "choose" how much force to exert on an object to pull it to the ground? Surely, if the same amount of force required to pull an army tank to the ground was exacted on me, I would be crushed? I'd greatly appreciate if anyone could provide any answers.
L. E. Gant2013-07-26T16:14:16Z
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The force depends on the masses involved. So, a large mass is attracted to the earth more than a small mass.
The magnet stays in place, because it resists the acceleration towards the earth. So does the poster with its blu-tack. Both are still attracted to the earth, but stay put because the resistance equals the attraction downwards.
Gravity doesn't "choose" the force to exert -- it's more or less dependant on the amount of attraction.
Put you between the tank and the earth, and your body will not give quite the same resistance as a rock would...
Let w = mg and W = Mg be the forces of gravity on the blu-tack and the tank. The mass of the tank M is waaaay bigger than that of the blu-tack m <<< M.
w and W are used here for forces of gravity because we often call those forces the weights. When you step onto the bathroom scale to see how much you weigh, you are finding out what the force of gravity is on your mass.
The gravity field g on both m and M is g = GE/r^2 where E is the source of gravity mass (e.g., Earth) and r is the distance between M and E, and between m and E. Neither m nor M is in this equation; so g does not depend on the blu-tack or tank. Also note there is no...no...force or weight without some other mass like m or M for g to act on.
To use your terminology, g always chooses to pull harder on objects with the greater mass.
So there you are. When m < M and they are both subjected to the same gravity field g, we have mg < Mg so that w = mg < Mg = W and the weight w of the blu-tack is way less than the weight W > w of the tank.
The force of gravity, or rather the gravitational force exerted on an object, is the product of its mass and the gravitational acceleration of the Earth (Fg = mass x 9.81 m/s^2). You are not held down with the same force as a tank, your smaller mass is accelerating towards the center of the earth at the same rate, so the gravitational force exerted on you is smaller than on the tank.
If you were at a great height above sea level, the acceleration of gravity would be less even if your mass stayed the same, so the gravitational force acting upon you would be less. The further away you get from the centre of the Earth, the less gravitational force you feel.
Magnets are able to stick to the steel wall of a fridge because the magnetic attraction between the magnet and the steel is greater than the gravitational force trying to pull it down. It doesn't mean that the gravitational force is weak, since a magnet will still fall if you drop it, it just means that the magnetic force between those two objects is greater. Adhesives stick to walls because the adhesive bond is stronger than gravity.
Gravity doesn't "choose" anything; it's all about your mass and how far away from the Earth's core you are.
Gravity is a very WEAK force, but the earth, as compared to you and me, is very large. As you know (don't you), the larger an object is, the more gravity it has.
But understand that "all objects with mass" (as the scientists put it) have gravity, AND, the amount of gravity that an object has is directly proportional to its mass. The more MASSIVE an object, the greater its gravity.
Still, gravity is a WEAK force. MAGNETISM is MUCH stronger than gravity, so the magnet stays on the fridge. Blu-tak doesn't count -- it is a commercial product designed to overcome gravity. Given enough technology, ANYTHING can overcome gravity, from a piece of blu-tack to the engines of a Saturn V rocket.
ALSO understand that ALL object with mass have gravity. So not only is the earth attracting you and me towards it, WE are attracting the earth towards us! And THAT is how BOTH YOU AND THE EARTH "decide" how much gravity to use. The gravitational attraction between two objects with mass is determined by the two masses.
So, !NO! -- the amount of gravitational force used to attract an army tank IS NOT the same as the amount of gravitational force used to attract you, but if you were to stand on a bigger planet (like Saturn??) which has more gravity, you would be squashed flat!
But gravity is ALSO affected by distance. The farther you get from an object, the weaker its gravitational effect on you. This is why we can stand on earth, but not float away into space towards the sun. The sun is MUCH large than the earth, and has many time more gravity, but since we are MUCH farther from the sun than from earth, we are attracted to the earth and not the sun.
[When two objects, like the earth and the moon, are orbiting each other, there exists a point called the Lagrange L1 Point. At this point, the gravity from the earth (over distance) is exactly the same as the gravity from the moon (over distance). If an object (like you or me in a space suit) were to be at that L1 point, we would stay there forever, being pulled by gravity to NEITHER the earth or the Moon. This is over simplified, but that is how it works.]
This is the very reason why planets stay in orbit. While the sun attracts US, we also attract the sun! The relationship between the mass of the earth and the mass of the sun determines how much gravity is "needed" to keep us in orbit, AND determine what size our orbit is.