# What is gravity?

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But if a massive object can create a warp in space time, the body orbiting it, wont that body eventually hurdles towards that massive object? For instance, the second link you psoted, in that video don't you think that the orbiting object just end up coming in contact with that one it is orbiting?

If i place a bowling ball on a soft mattress and it creates a warp in it, and i then go ahead and place a smaller object near the edge of where that warp begins, the smaller object will just head towards the bowling ball?

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Not really rigput. You forget that all massive bodies are capable of creating a singificant warp in spacetime. So for example even our moon can create its own warp. Essentially therefore you have the moon running along a groove that it cuts out caused by its own warping of spacetime as it moves through space inside another larger dent caused by the Earth's warping of space time. So essentially the moon is 'held in place' by its own dent in spacetime. This is again repeated on the scale of the solar system, where you have whorls within whorls caused by each of the planets and their moons as they rotate inside the much larger parabola caused by the warping of spacetime by the Sun - and this pattern is once again repeated both on a galactic and a universal scale too.

The reason therefore your smaller object fell towards your blowing ball on your soft mattress (as all such objects invariably do) is because it simply wasn't big enough to create a significant dent in spacetime to prevent it from doing so. (Although all objects do do this to a greater or lesser extent).

Try placing a shot put or any other smaller heavy object next to you bowling ball on your mattress, you will quickly see then that your shot put isn't likely to roll towards your bowling Ball any time soon - at least not without considerable force. If you scale this up to the scale of planets and moons and starts, you will quickly see how this force can become very significant.

GJ

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In a sense, the objects ARE accelerating towards the body creating the bigger dent. A moon in an orbit around a planet is experiencing a force towards the centre of the orbit (produced by gravity, or the dent if you look at it like that). This force is known as the centripetal force. As the moon orbits the planet, it is accelerating towards the centre of its orbit (It is constantly moving perpendicular to its current motion and hence Change in velocity = acceleration, it is accelerating).

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Lol, thanks for the clarification. Just trying to keep things simple enough so that everyone can follow. :) But yes this is correct.

According to Newton's law of inertia, an object already moving will continue to move in a straight line at a constant speed unless acted on by an outside force. Thus, to make an object move in a circular path, an outside force must act on the object. Centripetal force is the force that pushes or pulls an object inward so that it will move in a circular path. The word centripetal means to seek the centre.

When you whirl a stone tied to a string in a circle, you must constantly pull on the string to keep the stone from flying off in a straight line. The force the string applies to the object is the centripetal force. Centripetal force operates in other scenarios as well, acting, for example, on a car to make it travel around a curve. In this case, the force is produced by the friction between the tires and the pavement. Likewise, Earth's gravity exerts a centripetal force on a communications satellite that prevents it from flying off into space by keeping it in orbit.

If therefore you were to imagine a cylindrical container, as the ball rolls around the inside of the cylinder walls, a centripetal force continually redirects it to the centre of the cylinder. If you were to create an opening in the cylinder through which the ball could escape, then clearly the ball would fly outward along a line tangent to the cylinder and would then continue in the same direction of travel as at the moment it exited the cylinder. When this container is held upside down, the centripetal force is not strong enough to counteract gravity, which pulls the ball downward and out of the cylinder.

With a second container, a pitcher with a wide body that curves inward near the lip, the ball does not drop out of the opening. As with a cyclist riding on a banked racetrack, the downward force of gravity is matched by the upward force provided by the surface over which the ball (or the cyclist) travels. Centripetal force acts on the ball as it rolls around inside the pitcher, putting the ball in position to be supported by the pitcher's curved surface, which is what really keeps it from falling out. so in this sense gravity and centripetal force simply balance each other out. This is also what keeps motorcyclists in place on the old style 'wall of death' rides that you often find at fun fairs and fairgrounds).

This is all elementary physics though, so if you are really interested a good place to start would probably be some beginner level physics books, or if you are still at school, perhaps you could even do a course? Even if you are a little older the best way I think to learn is to read and perhaps try making some experiments fun by doing the with your kids, to see what results you get. This way they get to learn - and you learn something new and interesting too!

GJ

Edited by raid517
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Im at University doing Physics :blush: - was just trying to explain to ripgut why the body wouldnt fall straight towards the object.

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Im at University doing Physics  - was just trying to explain to ripgut why the body wouldnt fall straight towards the object.

Lol thanks, I've read some of your stuff before and you really do give some great input.

You certainly do seem to know your stuff. ;)

GJ