Final answer:
A 5 kg block on a horizontal frictionless surface is governed by Newton's first law and will remain in its state of motion, whether at rest or moving at a constant velocity, until an external unbalanced force acts upon it. The only forces present are gravity and the normal force, which are balanced and do not influence the block's motion.
Step-by-step explanation:
When a 5 kg block moves in a straight line on a horizontal frictionless surface, it is subject to Newton's first law of motion. This law, also known as the law of inertia, states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external unbalanced force. In the absence of friction, if the block is at rest, it will stay at rest. If it is in motion, it will continue moving at a constant velocity indefinitely, as there is no unbalanced force to change its state of motion; this includes both speed and direction.
The only forces acting on the block in such a scenario would be the gravitational force exerting a downward pull (which is equal to the mass of the block multiplied by the acceleration due to gravity) and the normal force exerted by the surface, which exactly counteracts the gravitational force. These two forces are balanced and do not change the block's motion.
Therefore, on a horizontal frictionless surface, unless acted upon by an external unbalanced force, the block's motion remains constant, exhibiting zero acceleration.