Question

cool question.An helium ballon is fixed in a wagon floor with a massless thread,the wangon is completely closed.The thread stay in vertical position while train moves with contant velocity.If the train is accelarated forward ,we can say,in relation to the the train the ballon will:

a)move back and the tension in thread will increase
b)move back and the tension in thread not change
c)move forward and the tension in thread increase
d)move forward and the tension in thread not change
e)remains in vertical position