MadSci Network: Physics

Re: What is the evidence that gravity is a pulling force?

Date: Wed Jan 7 10:12:15 2004
Posted By: Michael Wohlgenannt, Grad student, Department of Theoretical Physics , University of Munich, Germany
Area of science: Physics
ID: 1073180960.Ph

Hi Jonas,

thanks for posting this question. The gravitational attraction of the Earth keeps you on the surface and prevents you (and the atmosphere) from floating out into space. If you jump up, you exert a force which accelerates you upwards. But the gravitational force is alway present and is always pulling you towards the Earth. You accelerate upwards and the gravitational force is decreasing your acceleration. Finally it is even accelerating you back towards Earth, and you will drop down again. To evade the gravity of Earth you have to reach the socalled "escape speed", which is 11,3 km/s.

The evidence that gravity is a pulling force, is the success of Newton's theory.

However, Newton's law is limited, as any law is. In some circumstances it has to be replaced by Einstein's theory of relativity. There gravity is no force at all, but a geometric effect of curved space-time. Newton's law is recovered for weak gravitational fields, rather small scales and speeds.
In some circumstances, even Einstein's theory seems to fail, and it has to be replaced by a yet unknown theory of quantum gravity. So the real nature of gravity still lies in the dark. But in the realm of Newton's mechanics, every two bodies in space attract each other by the gravitational force. This force is directed along the line connecting the centers of the involved bodies involved.

I hope I could help you,

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