so i saw a question unit 1 GCE may 2015 17 (b)
it required us to explain why holding on to a vertical stand would prevent the passenger falling backwards, so,the answer talked about newton's 3rd law first because as the arm acts on the train the train acts on the arm,3rd law pairs are supposed to be of equal magnitude,im wondering why they said there WAS a resultant force because the passenger was accelerating with the train. Where did the extra force come from? Cause if the forces are of the same magnitude,then why cant we say theyre cancelling out?