According to the first law of newton,eveybody continues to
be in its state of motion or uniform motion in asraight line , unless compelled bysome
external to act or otherwise. This imlies that the acceleration of the body is zero when
the net forces is zero. The acceleration could be there if only there is some external
force is acting on the body.
In case of a spaceship when
it is in space free from all gravitational forces, if suddenly the rockets are turned
off, the spaceship will not have acceleration, but still it continues in motion with a
uniform velocity. To bring it to the zero velocity we must give it a breaking force by
turning on the rocket.
Sometimes we feel that an object is
at rest and we may feel that there is no force acting on it. An example is a book on
the table.It is at rest because it has two equal forces acting in opposite directions-
force of its own weight due to gravity pulling it down and another force of normal
reaction by the table acting on the book.
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