Originally Posted by
Unknown008
The first thing that came to me on seeing this question is that the reason was that the net force is zero. Well, not at the beginning but later on, when the cart no more accelerates.
Firstly, the horse 'uses' friction of the ground to move forward, and since there is no net force on the cart, it accelerates in the direction of the pulling horse.
Now, friction builds up as the cart gains speed, and the frictional force becomes equal to the force exerted by the horse.
Finally, as there is not net force (weight is cancelled by Earth's reaction, forward force by friction) the cart still continues to move. This is because "any body continues in its state of rest or motion in a straight line unless acted upon by a force" by Newton I.
Anyway, if that possibility is wrong, someone tell me why, so that I understand it better. I'm too in Grade 11, well I think it's the equivalent of what I'm doing.