A bus is moving and lots of ppl jumping

[darkar]

A bus moving with constant velocity. Passengers inside all jump at once, is that external force or internal force?

THx.

 

[daniel_i_l]

It depends - if you want to look at the force exerted on the entire system (people and bus) then it is an internal force. If you're only looking at the bus then it is an external force. It is important not to think that if the passengers are physiclly inside the bus then it must be an internal force no matter what.
For example, it might help to imagine the bus as simply a platform with people standing on it.
A force is internal if whatever that's exerting the force is part of the system. You can define what the system is however you want, depending what your trying to figure out.

 

[darkar]

I see. Thanks ~