A body continues in its state of rest until it is acted upon by an unbalanced external force.
According to Newton's first law of motion, the reason for motion is the action of unbalanced forces. A body will continue in its state of rest or uniform motion unless it is acted upon by an unbalanced external force. This law also referred to as the law of inertia.
Hence, an object will continue moving uniformly on a plane friction-less surface till the resultant force acting on it is not zero according to this law. Thus, the reason is an explanation for the assertion.
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