# I just don't get it

gwynethrosie
I'm a junior in hs & I'm in my 3rd week of Honor Physics & I'm struggling already. We're currently working on a chapter focusing on Kinematics in One Dimension & we're being tested on Thursday. Not only am I worried about the test, but I have a huge packet of problems due Thursday as well.

I think all the problems are pretty basic, but I just don't know how to start them. The problem that I'm working with right now says: A plane is sitting on a runway, awaiting takeoff. On an adjacent parallel runway, another plane lands and passes the stationary plane at a speed of 45 m/s. The arriving plane has a length of 36 m. By looking out of a window (very narrow), a passenger on the stationary plane can see the moving plane. For how long a time is the moving plane visible?

All I have down on my paper is a drawing of two planes and the numbers listed in that problem. If anyone could tell me where to go next, I'd appreciate it!

Thanks!
Gwyneth:)

## Answers and Replies

moose
If the person viewing out the window is stationary relative to the runway, and the landing plane is at a constant velocity of 45m/s, try to figure out what exactly is affecting the time. In this case the only thing moving is the other plane at 45m/s. If the plane is 36m, and it passes at 45m/s you should be able to calculate this very easily.

I made a lot of assumptions because the questoin did not specify on acceleration, etc.

Science Advisor
Homework Helper
I don't particularly like that problem! We are told that the arriving airplane has length 36 m but the only thing said about the window is that it is "very narrow"! And my experience with even narrow "airplane" windows is that if you lean very close to them you can see aways down the runway. Okay, I'm just going to ignore my quibbles.

Assuming you can only see straight through the window, the airplane will be visible through the window from the time the front of the airplane passes the back edge of the window until the end of the airplane passes the front edge of the window. Assuming this "very narrow" window has no width at all, that is from the time the front of the airplane passes the window (a single point) until the end of the airplane passes the same point- and the airplane is 36 m. long so the front of the airplane (as well as the back) must move 36 m to bring the back of the airplane to the point the front was at. How long does it take the airplane, at 45 m/s, move 36 m? You know that v= d/t so how would you find t, given v and d? Or: how do you combine "m" and "m/s" so that the "m"s cancel and leave "s"?

gwynethrosie
thanks :) i figured it out!