Imagine a body is kept on a surface with zero friction and no force is applied to the body. So will the body start moving?? I guess it shouldn't according to Newton's first law it will continue to remain in it's state( at rest ).
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$\begingroup$ Without force the body will not experience any acceleration (it won't speed up or slow down). Hence its velocity will not change; it will keep any velocity it had. $\endgroup$– hdhondtCommented Aug 27, 2019 at 3:08
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$\begingroup$ It can move. But only if it already was moving. It cannot start to move or otherwise change its speed without a net force. This is Newton's 2nd law. $\endgroup$– SteevenCommented Aug 27, 2019 at 7:25
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4 Answers
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Imagine a body is kept on a surface with zero friction and no force is applied to the body.
This cannot happen on earth, because a continuous force is applied, gravity . You have to state : horizontal surface. On such a surface the force of gravity is in a direction perpendicular to it , and its effect on the motion will be zero.
So will the body start moving?? I guess it shouldn't according to Newton's first law it will continue to remain in it's state( at rest ).
Only in this case, the horizontal surface, will it remain at rest if it is at rest. The slightest force in the direction parallel to surface will give it an impulse and it will move with constant velocity. On earth this could mean small earthquake tremors, or a truck passing on the street outside,
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As hdhondt said in his comment, if the surface is flat then the body will not move. It can be moving at a constant speed and it will continue moving. To change the speed of the body will require a force because of F=ma. Changing speed requires acceleration or deceleration.
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The body will not move if it is initially located at the tangent point between the flat plate surface and the earth as per the other answers. In other words, at the point where a line from the center of mass of the earth is exactly perpendicular to the flat surface. However, if the body is initially located to either side of the perpendicular line a new line drawn from the center of mass of the earth thru the body will not be perpendicular to the flat surface(at long and longer distances). In effect the body at this location will be at a higher elevation and will want to move to the lowest point which is the perpendicular point. Of course the plate could be rotated around the earth center so as to become tangent again. Its kind of like trying to balance a marble on a bowling ball. It looks good on paper. If the mass of the earth did not create a gravitational gradient then the body could be located anywhere on the plate and not move but there would also be nothing to hold it against the plate in the first place.
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By law of inertia, we know that a body will remain in its state of motion or rest until compelled by an external agency. Here you have assumed such a situation where there is no external force ( assuming that it is not possible on Earth but possible in vacuum) then the body will remains in its state ( whether it is in motion or rest) for ever.