If you are on a moving bus why do you stay in the same place if you jump up?

Answer 1

According to Newton's first law of motion, an object in a state of uniform motion will tend to stay in that state of motion until acted on by an external force. So, using this principle let's consider the case of a person on a bus. When you are standing on the bus, you are moving in the same direction with the same speed as the bus. When you jump up, the only force acting on you is gravity, which only affects your up-down motion. Assuming the bus is moving on a flat surface then, there are no forces acting on you in the direction of the motion of the bus to change your motion. Thus, you keep the same motion that the bus has, and you will stay in the same place relative to things inside the bus. There are situations in which this wont work! Consider, for example, what would happen if the bus made a sharp turn after you had jumped into the air. In this case, the some force has caused the bus to change direction, and so change its motion. However, since you are in the air, this force doesn't act on you. The result is that you will feel like you are moving relative to things inside the bus! Another related example of Newton's first law is when you jump on an elevator. If you jump just as the elevator is reaching the top, you will feel like you have jumped very high! This is because when you jump, you have the upward motion of the elevator, in addition to your jumping motion. When abraking force acts to slow the elevator down as it's reaching its destination, you avoid that force since you're in the air.