Marbles slow down due to friction, which opposes motion and causes a loss in kinetic energy. To move fast again, an external force must be applied to the marbles to overcome friction and increase their kinetic energy.
In order for marbles that have slowed down to move fast again, they need an application of external force. This force is needed to overcome friction and increase the kinetic energy of the marbles. Friction is a force that always opposes the motion of objects, and it causes them to lose kinetic energy and eventually come to a stop. When the marble was initially let go at the top of the pipe, it had high gravitational potential energy and zero kinetic energy; as it fell, this potential energy was converted to kinetic energy, causing it to move faster. To make the marbles move fast again, one must either push them or allow them to fall from a height to convert gravitational potential energy back into kinetic energy. In an experiment to observe simple harmonic motion (SHM), the restoring force of gravity plays a crucial role, and the force you apply assists in maintaining the motion.
Similarly, the TAKE-HOME EXPERIMENT involving the marble dropped in various fluids demonstrates the effect of viscosity as a form of fluid friction, affecting the terminal speed and thus kinetic energy of the marble. When comparing different smooth surfaces, the rougher the surface, the more friction there is, and hence the slower an object will move, requiring more force to maintain the same speed. Galileo's insights reiterate that without such opposing forces, an object in motion would continue indefinitely. Therefore, to recap, for marbles to move fast again, one needs to apply an external force to compensate for friction and restore the kinetic energy of the marbles.