Answering 6th Grade Physics: Explaining School Bus Jerk Using Newton's Laws

[shayes928]

The following question appeared on my son's 6th grade physics test recently:

"Why are passengers on a school bus jerked backwards when the bus comes to a sudden stop? Explain using Newton's laws of motion."

I was just wondering what a good answer to this might be.

Thanks in advance!

 

[Chi Meson]

6th grade?

The statement you wrote above is wrong though. The passesngers on the bus will NOT be jerked backward when the bus comes to a sudden stop.
Either they will be jerked forward when the bus comes to a stop (assuming the bus was moving forward to begin with), OR they will be jerked backward when the bus suddenly starts.


The answer lies in Newton's First Law of Motion: (Sixth grade version)
An object at rest will remain at rest unless a force causes it to move; AND an object in motion will remain in motion (at the same speed in a straight line) unless a force causes it to change its motion.

The passengers are not really "jerked." They simply remain in motion while the bus stops, or they remain at rest while the bus starts. The passengers are waiting for the bus seats to exert the force that will change their own motion.

 

[Njorl]

Because this schoolbus was traveling in reverse!

They are jerked backwards by a sudden start, not a sudden stop. Objects in motion tend to stay in motion; objects at rest tend to stay at rest. When the bus starts moving, the children tend to stay at rest, and are thus jerked back relative to the bus.

Njorl