My teacher never went over this in class

[RyanJF]

Homework Statement

"A beam of electrons in a television tube moves horizontally with a velocity of 1.00 x 10^7 m/s. How far will the electrons drop as they travel a horizontal distance of 20.0cm?"

Vx (final velocity) = 1.00 x 10^7
Vox (initial velocity) = 1.00 x 10^7
Ag = 9.8
Xy (vertical distance) = ?

Homework Equations

Equations of kinematics

The Attempt at a Solution

My teacher never covered using horizontal (x) components to get an answer for something in the y-component. Therefore, I'm a bit confused as to what do in order to get the answer for how far the electrons dropped.

I just used a standard equation (X = 1/2 (Vo + Vf)t) and got some wild answer that I know is not correct. So instead of asking for an answer, or for a check, I just want to know what process I need to use in order to get the right answer.

Maybe I shouldn't major in physics once I get into college, regardless of how much I like it.

 

[zgozvrm]

#1 - How long will it take to cover a horizontal distance of 20.0 cm at a horizontal rate of 1.00 x 10^7 m/s?

#2 - How far will anything drop in that amount of time?

 

[RyanJF]

Gah, I feel like such an idiot - I was about to post that I already solved for T, getting an answer of 2.02 seconds. Then I realized that my answer would be in meters, but I used the centimeter value of X to calculate T.

I suppose that my "T" would then be .202 seconds, not 2.02. What equation could I use to solve for displacement in the Y? All I'd have to do is 1/2 * 9.8 * .202^2, right?

Sooooo, assuming that I did that all correctly, the electrons dropped a whopping .19796m. Uh, that's a big television tube, unless I'm wrong again.

 

[RyanJF]

Somewhere along the way, I think that I forgot that an object dropped vertically will travel the same distance in the Y-component as an object thrown vertically. It'd be nice if somebody can check my answer, though.

I caught that one on my own, but I still have to thank Zgozvrm for making me look back at my time equation!

 

[zgozvrm]

How did you come up with 0.202 seconds? (That's way off...)

 

[RyanJF]

T = sqrt(2x/Ag) = .202 seconds

I checked it with another equation for acceleration, and it's right. We went over it just a few minutes ago, and I did all of the work correctly.

I won't say that I'm sure beyond a doubt, but what do you think that it should be?

 

[zgozvrm]

The horizontal distance is 20 cm which is 0.20 m.
The horizontal velocity is 1.00 x 10^7 m/s = 10,000,000 m/s

V x t = d
so, t = d / V

This is the time necessary for the electrons to travel 20 cm (note that the acceleration of gravity doesn't affect the horizontal velocity).

 

[RyanJF]

Hm. That definitely makes more sense. I thought that the answer of .1999m seemed rather large for a television tube, although I'm still feeling stupid for not getting that answer right off the bat. As I said, maybe considering a physics major is a bad idea, at this point in time.

That solution seemed obvious enough. I don't know how, or why, my teacher got the same answer as me, though. That looks pretty bad for him.

 

[zgozvrm]

I don't know how, or why, my teacher got the same answer as me, though. That looks pretty bad for him.

Everyone makes mistakes. It sure makes it hard for the student when the teacher (or the book) is wrong!