Final answer:
The net force on the object must be zero.
Step-by-step explanation:
When an object is in motion and its motion does not change, it means that the object is moving at a constant speed in a straight line, known as uniform motion. According to Newton's first law of motion, an object will continue to stay in uniform motion unless acted upon by a net external force. Therefore, if the motion of an object does not change, it implies that the net force on the object is zero.
For example, if a car is moving at a constant speed on a flat road and there are no other forces acting on it (like friction or air resistance), the motion of the car will not change because the net force on the car is zero.
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