why we should not jump into or out of the running bus or a train

QUESTION- Why we should not jump into or out of the running bus or a train

ANSWER- According  to newton If a body is at rest it remains at rest unless a external force motivate the body to change position that property of a body is known as inertia (Resistance to change)


Now lets come to question 



since our body is at rest as we stand on ground but train or bus is moving so according law of motion our body's inertia resists our body to change state. means inertia doesn't  allow us to shift from rest to moving body as there are speed differences 

But in case of we are in train then our speed or our state is moving but ground is at rest so sudden change of state can cause some serious injuries 

conclusion - Inertia is the only thing that doesn't allow us to change state between two different surfaces