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take the example of earth sun system. The earth is said to orbit around sun because the sun bends the space time around it .

Now if by some means we stop earth in its path such that its at rest with respect to sun then release it , it will immediately start a free fall towards the sun .

Now the question is if the gravity is just bent space time then what is causing the earth at rest to move from rest. What causes the force?

how can you explain both the Einstein bending of space and newtonian force of gravity existing simultaneously?


marked as duplicate by John Rennie general-relativity Aug 22 '16 at 10:58

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There is no force on a body falling towards a star. Rather than considering some obviously hypothetical / mad thing like dropping a planet (having first stopped it in its orbit, even more madly), consider dropping a person: do you experience any force in free-fall? Well, then: there is no force, you are just following a geodesic. Remember that there are many geodesics through any given spacetime point: it's not the case that an orbiting body is somehow on the only possible trajectory through a given spacetime point: there are others (indeed an infinity of others), some of which are those which are you falling towards the Sun.

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    $\begingroup$ You donot feel force of free fall in gravity because every particle in your body is accelerating at same rate due to gravity at all time during the fall. This is different from acceleration in a car where an external force enters your body and affect different parts over time as it travels through. whereas gravity acts instantaneously on all of your body hence you don't feel it. Even if you put all that crazy talk aside we all know accelration of a body with mass directly implies presence of force $\endgroup$ – user1062760 Aug 22 '16 at 18:57
  • $\begingroup$ @user1062760 A body in free fall is not accelerating. $\endgroup$ – tfb Aug 23 '16 at 19:11
  • $\begingroup$ Only valid if you take general relativity explanation , the newtonian model would become invalid if you say things like "A body in free fall is not accelerating" , in newtonian mechanics the body does indeed accelerate and a far observer can see you fall faster and faster due to acceleration. $\endgroup$ – user1062760 Aug 24 '16 at 4:46
  • $\begingroup$ @user1062760 Well, yes. Unfortunately Newtonian gravity is experimentally wrong, and people prefer to use theories which are less wrong. $\endgroup$ – tfb Aug 24 '16 at 7:39

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