Expert: Daniel Mazur - 12/17/2005

Question
Hi,
I hope I'm finding you in the best of health.
I am 16 years old and in high school
By projectiles, when throwing something horizontally(as opposed to at
an angle), why is the initial velocity of the y-component zero and the
x-component is not zero? Don't they both start from rest?
Also, does this mean that whenever someone throws something, that
there will never be any acceleration?

Answer
Hi Jeff,
Thank you for your question, I will gladly answer you.

The confusion you feel arises from the textbook tendecy to give you simple model(!) problems to solve, rather the complicated ones that real life brings. For example, in the real life when something is being thrown, it of course experiences acceleration in the direction of the throw, as long as the acting person is still holding it. Only after he/she lets go, from that instant it is only velocity of the object and gravity that matter. And of course in the real life  the vertical component will never be identical zero, it will only be close to it...

Scientists work with model situations and they will usually start with a relatively simple model, which will solve a problem aproximately, or only in some special conditions. After such model is created, it is tested and if it fails the tests of real life, there comes the refining stage. I will use your example to demonstrate.

In the simplest possible model it does not matter, how the object had gained its velocity. We simply put our time=zero "stamp" on the instant right after the object has been released (i.e. the forward acceleration has gone, only the velocity state remains). In such model, we can omit the air friction and we can disregard that the velocity may have some small vertical component. It will create some error in our model, but we cannot expect a simple model to be almighty, can we?

Well, upon testing we might find it useful to extend our model to throws at an angle (that will dramatically increase the success of our model, while very little extra effort was necessary). Maybe later we will be forced to include air friction in our calculations, which will make the model even more precise, but already at the cost of becoming quite complex - beyond high-school curriculum I dare say. Same thing goes to the case, if we start studying the effect of different ways, how the object may have been accelerated before time=zero. It is more trouble than it is worth and at a high-school level all this would make classes a terrible mess.

So, textbooks give you examples and problems based on the simple models I mentioned. It is their (and the teachers') job to explain students that we are all, of course, aware that real life is much richer than textbook problems. The textbook presents us with only pieces that are easy to grasp with calculus than the real life situations. When you throw a ball at someone, you will ALWAYS accelerate the ball, while its is still in contact with your hand. In the instant the ball leaves your fingertips the high-school physics says "NOW!" and FROM THIS POINT ONWARD ONLY the high-school physics will be able to make predictions as to where the ball is going to land.

I hope I have brought some light into the matter for you. May it serve you well, good luck!

Daniel