Topic: Newton's First Law of Motion.
Why do you fall backwards when a bus starts suddenly?
Did you observe that the bag fell when brakes were applied?
Do you know why this happened?
This happened because of inertia.
Inertia is the tendency of a body to be in a state of rest or in a state of motion.
Newton's first law of motion defines inertia.
It states that a body in rest stays in rest and a body in motion stays in motion.
Unless it is acted upon by an external force.
So, in this case, when the bag was on the bike, it was moving along with the bike.
Now, when brakes were applied suddenly.
The bike stopped but the bag kept on moving forward due to inertia of motion and hence it fell.
This time the bag fell again.
Can you guess why?
Yes, you guessed it right.
It is also because of inertia.
When the bag was on the stationary bike, it was at rest.
Now, when the bike was accelerated suddenly, it moved forward.
But the bag being at rest, did not move due to inertia of rest and hence it fell down again.
In this case, when the bus suddenly starts moving forward.
Our feet which are in contact with the bus start moving forward.
As a result, the lower part of our body moves forward.
But the upper part of our body, which is not in contact with the bus still tends to remain in a state of rest.
The upper part of the body does not move forward along the lower part.
That is why we tend to fall backwards.