Final answer:
According to Newton's first law of motion, an object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by an external force. To make an object slow down, an external force in the opposite direction of its motion, such as friction, is required.
Step-by-step explanation:
According to Newton's first law of motion, an object will continue moving at a constant velocity unless acted upon by an external force. To make an object slow down, an external force in the opposite direction of its motion is required. This force is called friction. Friction can be caused by various factors, such as air resistance or contact with a surface.
Learn more about Newton's first law of motion