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According to the law of conservation of linear momentum, the momentum of a closed, isolated system remains conserved when no external force acts on the system.

For example, consider a system of two balls that collide head-on. If no external force acts on the system, the total momentum of the system of two balls before the collision is equal to their total momentum after the collision.

Thus, the total momentum of the system remains conserved.

The momentum of a moving object remains conserved when it is not acted upon by any external force. However, in the real world, external forces like frictional force, gravity, and force due to air resistance act on the object.

Therefore, momentum does not remain conserved for the object acted upon by external forces, and hence it eventually slows down and stops.

Consider an example of a ball moving on a horizontal plane floor. When the ball is moving on the floor, the air present in the atmosphere applies its force on the ball to resist the motion of the ball.

Also, friction is present between the floor and the ball. Therefore, frictional force will act on the ball in the direction opposite to the motion of the ball.

Under the combined effect of frictional force and air resistance, the motion of the ball is opposed. Thus, the ball will slow down and eventually will stop over time.

However, if the same ball moves on thefrictionlessfloor inside a vacuum chamber, neither of these external forces will act on the system, and the ball will not stop over time. Thus, the law of conservation of momentum will be obeyed.

So, the momentum of a moving object is conserved when no external force acts on the system. Thus, it can be clearly said that most moving objects slow down and stop because of the external forces acting on them.