Sticky

#### By chance are any of you physics teachers?

MOD
Because if you are, I need help, a lot...
not me.. but i'm good at the math if you've got the formulas
I have a math degree... but I couldn't do physics to save my life.

Jason
I got a D- in physics. But if its any help, I was born in the same town as the guy who wrote the book we used that semester.
MOD
One of my problems is I'm not sure what formula I need to use. This the problem I'm stuck on right now.

While riding on an elevator descending with a constant speed of 3.0 m/s, you accidentally drop a book from under your arm. A) How long does it take for the book to reach the elevator floor, 1.2m below your arm? B) What is the book's speed relative to you when it hits the elevator floor?
MOD
that may be why you squeaked by w/ a d-

:/
MOD
ummm..... crap.....
now, I could be very wrong here.. I would think that for A) 9.8m/s^2, it falls 1.2m. so 1.2m/9.8m/s^2=.1224s^2 = .3499s could be how long it fell. though it is already going 3.0m/s... so I'm not positive. as for B)speed = v+at ... so 0+9.8*.3499 which would make for 33.6044m/s^2

I'm not really sure how the 3.0m/s fits in.. if it does at all, physics likes to trick you like that. What i always used to do in that kind of problem was write down every piece of information they provide in a nice list, then write the basic formulas for what I'd need to find. In this case time and velocity. then I always found it easier to figure out which formulas would work for the information I was given. Its harder to do that with the internet as a formula sheet though... I hope this made sense, I'm rather sleep deprived at the moment. Nothing as exciting as physics though.. computer ethics for me ><
Hi,

The velocity relative to the earth (that is, the speed of the elevator relative to the earth plus the speed of the book relative to the elevator) is immaterial to this problem because it doesn't ask for that information. As shoddy shot said, this information in this problem is there to trick you. Imagine that you are doing this problem with you dropping a book in the backyard or something.

Then it comes down to using one of the four kinematic equations for uniform acceleration... whichever one you can fill in the best with the variables that you know (initial velocity of 0, acceleration of 9.81 m/s^2, distance 1.2m). You will probably need one equation for the first part of the question and a different one for the second part of the question.

Also, don't forget to draw your free-body diagram for full credit :-).

Jason
***** see below *******

Well, guys, Karl may have looked at the wrong place for advice here. Think of it this way, if the elevator was moving at a rate of 100m/s, you and your book would be pressed up against the ceiling so to say that the movement of the elevator makes no difference is in error.

What is DOES influence is the actual distance the book drops. That 1.2 meter drop under a "free fall" is actually more than than 1.2m because the elevator has moved down in the time that the book has taken to hit the floor.

Use d=vi(t) + (1/2)at^2, but in this case the d is actually 3t + 1.2m (don't forget that vi=3m/s)

Unfortunately, it looks like that turns it into a quadratic equation, but you can certainly use the quadratic formula to solve for your time.

Yes, I have taught Physics.
"That 1.2 meter drop under a "free fall" is actually more than than 1.2m because the elevator has moved down in the time that the book has taken to hit the floor."

The would only be true if the you and the elevator had a different initial velocity than book, that is, if the elevator is caught mid-acceleration. In that case, the book would need some time to "catch up" with the elevator as it is a body at rest with a force being acted upon it (in this case, friction and the normal force from the accelerating elevator).

The problem states that the elevator is already moving at a constant speed of 3.0 m/s relative to the ground. Therefore, the book, inside the elevator has a constant speed of 3.0 m/s relative to the ground. Since the elevator and the book have the same velocity (3.0 m/s going downward), they have a velocity of 0 m/s relative to each other.

Jason
But the elevator does not provide any normal force since it is not accelerating, it is going a constant 3m/s. As you drop it, the only force acting on it is gravity (and air resistance, which we can ignore in this example).

If the elevator was fallling at the same "free fall" as you and the book, the book would not hit the floor at all (until you crash at the bottom floor, of course.) According to the way that I see your reasoning, the book would still fall at the same rate relative to the person in the elevator, no matter how fast the elevator is going.

I think, though, that I see the issue -- is the elevator a "closed' system, or is it an "open" system.

Let's say that the elevator is actually going faster than free fall. Yes, you would eventually catch up to the speed of the elevator, then be going the same speed, but you would never "fall" back down to the bottom of the elevator and feel like it is "normal"

Now you have me thinking differently, because your acceleration would end up with your overtaking the elevator (but only if air resistance is not counted.) And gravity does accelerate independent of current velocity, so I guess you are right and this is a closed system.

This is a good mental exercise for me, since it has been a while since I taught physics. Thank you for that!
I say, quit being so damn lazy and just take the damn stairs.
how long does the book take to reach the bottom of the stairwell you say?
By the way, Jason -- your original response said:

"but I couldn't do physics to save my life."

What is up with that?
I guess that ends up being a matter of context. I struggled through Mechanics in college. As you can probably tell, I have trouble setting up these physics problems... I know the procedure, but even with an FBD I can't ever really visualize them.

While upper division Math classes are certainly hard work, I never had the feeling of "I just don't get it" the way I got in General Physics I. I completed Physics I with a B and could finish the problems competently, but I never felt like I understood it.

Jason
Also, I totally wasted like a 1/2 hour of my employer's time solving that problem. That's called, "Me being competitive" :-). I think that's leftover from my Math education (where everybody was competitive).

Jason
MOD
That was one of the things that I was getting stuck on. Is it a closed system or an open system. So if it's a closed system then d=1.2 not 3t + 1.2 correct? so 1.2=3t +(.5)(9.81)t^2 correct?

Then for part B I'm lost, is it just saying that you'd both be traveling the same speed again or that it's asking how much distance or speed or something between 1.2m in the elevator?
I think that on the second part, if you know how much time has passed you can just use another equation that has a final velocity and time parameter.

Jason
MOD
well I had to turn it in, I think I came out in the end with .89s and b)8.7m/s

they're supposed to post the answers at like noon so we'll see what was what then
Well Karl I think you scared me away from PSEO.
Karl,

Do they give you solutions or just the answers?

Jason
Allison,

You've got to take college classes sometime... Why not do them while they're free? Physics is a pretty tough class.... many college courses are easier.

Jason
I second Jason's logic. College is crazy expensive, its really worth it to take the free classes. I graduated with 9 credits out of high school, I wish I did more :(
MOD
I just took a look at it, it gives your the solution and answers. I think I over thought the problem. Once I figure out his writing I'll tell ya.

"I already have 15 credits" : )
"College is crazy expensive,"

And you don't even really GET how crazy expensive it is until you graduate and have to work those student loan payments into your lifestyle. Oh my golly.

Jason
haha, right on Jason. I'm not looking forward to that day at all.