Be advised this is not a homework question, even though it is in regards to a physics book I purchased over the summer to get a headstart on my upcoming AP class. I was reading the chapter about acceleration, and the book seems to state that there is no such thing as horizontal acceleration. It gives the example of a cannonball moving equal horizontal distances per time interval. I know vertical acceleration exists because of gravity, but is there really no such thing as horizontal acceleration? Am I simply misunderstanding the book? Doesn't a cannonball undergo horizontal acceleration when it is being fired from a cannon? It goes from a state of rest to faster and faster state of motion because of being acted upon by a horizontal force, correct? I tried to reason this out for a day or two, but it still makes no sense. Hopefully you guys can clear it up for me :) Thanks for your help in advance.