# Homework Help: Projective motion problems

1. Feb 21, 2004

### holly

Projectile problems are about as much fun as projectile vomiting.

Okay, I am stuck on all the same kind of problem.

Something is thrown or fired horizontally. There is no air resistance. After "x" seconds, it has fallen how far?

Ex: A bullet is fired horizontally from the top of a cliff. Three seconds later, the ball has fallen a vertical distance of ???
And
A bullet (violent, aren't they?) is fired horizontally with an initial velocity of 300m/s from a tower 20m high. No air resistance, the horizontal distance the bullet travels before hitting the ground is ????

Is the gravitational constant coming into play again? I know the ball/bullet is falling, and it's going sideways, too. is this a d=vt problem? Do I just go back to my little table of how far things fall in free fall for how far it falls down? But what for the horizontal component? Is that the d=rt part?

Thanking you in advance for any help. I'm lost again. New chapter, new confusion.

Last edited: Feb 21, 2004
2. Feb 21, 2004

### Hurkyl

Staff Emeritus
When you do do 2-d (constant acceleration) projectile problems, all you generally have to do is to treat the vertical and horizontal directions as seperate 1-d projectile problems.

Sometimes one of these 1-d problems is solvable for what you want. Sometimes you need to use one of the 1-d problems to solve for change in time (the only thing which is always the same in both 1-d problems) which lets you solve the other 1-d problem.

3. Feb 21, 2004

### marcus

Re: Projectiles....*sigh*

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Q:Is the gravitational constant coming into play again? I know the ball/bullet is falling, and it's going sideways, too. is this a d=vt problem?
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A:yes the g constant is coming into play (for the vertical component or dropping part of it) and the d=vt is coming into play for the horizontal constant speed part

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Q:Do I just go back to my little table of how far things fall in free fall for how far it falls down? But what for the horizontal component? Is that the d=vt part?
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A: yes you go back to the table of falling for the falling part.
and yes the horizontal component uses d = vt.

use your table just as if it were dropped. 5 t^2

how long does something take to drop 20 meters? use 5t^2 and find out
how many seconds

then ask how far the thing can travel in that many seconds

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Hurkyl's answer is the right one. I am repeating the gist of it (in detail) for good measure.

4. Feb 22, 2004

### holly

Thank you to both Big Brains for the answer. I was unsure what the 1-d and the 2-d was about, so I very much appreciate the restating of the procedure.

5. Feb 22, 2004

### holly

Diff Kind of Help Needed

I put a post over in the Value Theory forum...need advice, youse all have helped me with physics problems, this is a moral one. I have all the tests the prof is going to give, do I use them?

6. Feb 22, 2004

### NateTG

Re: Diff Kind of Help Needed

I think that depends on what do you mean by 'do I use them.' If you mean memorize the questions and answers on all of the tests - that would probably be cheating. Looking at the problems to see if you should study more is probably not so bad.

A bunch of it also depends on how you got a hold of the questions in the first place.

7. Feb 22, 2004

### holly

I got the book at the only used bookstore in Midland. I bought it fair and square. I didn't know it was the place he was getting his tests when I bought it.

Evidently SOME people are saying there is no book, there is no me, I don't really have a crazy class at a cow-town college, and so forth, so since I don't exist, I guess I have no moral dilemma. But thank you for your suggestion I use it to study and not to memorize. If I existed, I would take your advice to heart. But I'm evidently just a madeup little nothing.