Gravitational Test Mass

Question:

A reference book says - when we are finding the field lines of a mass, we should use small test mass so that the original system is not distorted. What does this statement really mean? How does it look like in the 'original system' & the 'distorted system'?

Answer:

If we want to determine the effect on space of a certain object, in other words its gravitational field, we can do this by introducing another object into the vicinity. The other object though is going to have its own effect on the space around it and so possibly corrupt the measurement of the first object's field. To minimize this distortion, we make the second object small enough compared to the first that its effect on the space will be negligible. Look at http://mcanv.com/p1ns.html and the following page in the online course for help picturing this.

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