# Adventures of Martin the scientist (V) The force of the bus

One Monday, when Martin came back from school on the bus, he noticed something that he had never paid attention to before.

— Okay, but there's something I don't understand. Today he was on the bus almost asleep in a seat, and he suddenly stopped. From the impulse I almost ended up sitting on the ground! — exclaimed the boy.

His mother looked at him worried.

— Are you okay? Have you hurt yourself?

— I'm fine, but the people who were standing ended up all pressed against the driver — said Martín.

— I can't find why that happened.

— That's because of the law of inertia! — Martin's mother said.

— The law of inertia? he asked.

— To change the speed of an object, you need to apply a force to it. An object that is at rest or still is going to stay that way unless we apply a force to it. On the other hand, if it is moving, it will continue moving unless we apply another force to it — his mother explained.

— But I was still, why did I move forward? Martin asked confused.

— But you weren't resting — she said.

— Yes, I told you I was almost asleep — repeated Martín.

— It could be, but you were inside a bus that was moving. The force that made you stop was the same force that made you move.

— And what happens if there is an object in motion and there is no force to stop it? Martin asked.

— Then it will continue at a constant speed.