Finding time to go up one floor

[astrololo]

Albert goes upstair on a moving floor. When he let's the moving floor make him go upstairs, it takes him 30 seconds. When he walks while being on the moving floor, it takes him 10 seconds. How much time would it take him to go upstair if the moving floor didn't work anymore (So, you can only walk)

I have zero ideas what to do in this case. I think it might have something to do with relative speeds, but I'm not sure.

 

[Student100]

Albert goes upstair on a moving floor. When he let's the moving floor make him go upstairs, it takes him 30 seconds. When he walks while being on the moving floor, it takes him 10 seconds. How much time would it take him to go upstair if the moving floor didn't work anymore (So, you can only walk)

I have zero ideas what to do in this case. I think it might have something to do with relative speeds, but I'm not sure.

You should really keep the template and make an attempt at the problem, even if you have no idea.

Hint: What's the distance?

 

[insightful]

By "moving floor" I assume you mean "escalator." What basic distance-velocity-time equation should you use?

 

[astrololo]

You should really keep the template and make an attempt at the problem, even if you have no idea.

Hint: What's the distance?

Sorry, next time I'll use it. Distance is speed times time.

 

[astrololo]

By "moving floor" I assume you mean "escalator." What basic distance-velocity-time equation should you use?

Yes, it's an escalator. I think I should use Distance = speed times time

 

[Student100]

Sorry, next time I'll use it. Distance is speed times time.

Don't worry about the equations, you're not to the point of using any equations yet.

I'm asking what's the distance of the escalator. What is the problem trying to tell you physically.

 

[astrololo]

Don't worry about the equations, you're not to the point of using any equations yet.

I'm asking what's the distance of the escalator. What is the problem trying to tell you physically.

The problem doesn't provide any distance. In fact, the very next question is if I can guess the distance with the time only.

 

[Student100]

The problem doesn't provide any distance. In fact, the very next question is if I can guess the distance with the time only.

Does it need to? Does the escalators length change? What can you tell me about the physical distance between the two points?

 

[astrololo]

Does it need to? Does the escalators length change? What can you tell me about the physical distance between the two points?

Oh, the distance stays constant whatever the situation.

 

[Student100]

Oh, the distance stays constant whatever the situation.

Exactly. So if you picked an arbitrary value of 60 or 90 meters, would the answer change? Why don't you try that out.

 

[astrololo]

Exactly. So if you picked an arbitrary value of 60 or 90 meters, would the answer change? Why don't you try that out.

Well, depending on the distance, the time would vary. I mean, 10m is different from 1000m. But I think that the ratio of time would stay the same, regardless of the situation, right ?

 

[Student100]

Well, depending on the distance, the time would vary. I mean, 10 is different from 1000. But I think that the ratio of time would stay the same, regardless of the situation, right ?

The times fixed in this case. So irrelevant of distance the time is the same. Did you try those two arbitrary distances?

 

[astrololo]

The times fixed in this case. So irrelevant of distance the time is the same. Did you try those two arbitrary distances?

I got s= 1 m/s and 1/3 m/s for distance 10 m.
s=10 m/s and 3,33333... m/s for distance 100m.

Not sure if this is what you're asking for...

 

[Student100]

I got s= 1 m/s and 1/3 m/s for distance 10 m.
s=10 m/s and 3,33333... m/s for distance 100m.

Not sure if this is what you're asking for...

If you had did 60 and 90 you'd get easier stuff to work with. :P

Anyway, now what can we assume about the velocity? Is there an acceleration or is it constant? And what you can you say about the difference between just the escalator and the escalator and walking? Is it possible to determine walking speed and then find the time?

 

[Student100]

I got s= 1 m/s and 1/3 m/s for distance 10 m.
s=10 m/s and 3,33333... m/s for distance 100m.

Not sure if this is what you're asking for...

You already have the answer, you just need to see it.

 

[astrololo]

If you had did 60 and 90 you'd get easier stuff to work with. :P

Anyway, now what can we assume about the velocity? Is there an acceleration or is it constant? And what you can you say about the difference between just the escalator and the escalator and walking? Is it possible to determine walking speed and then find the time?

Well, I get 2 m/s and 3 m/s for 30 seconds only. (This is the speed of the elevator, so the elevator is constant) I get 6 m/s and 9 m/s for 10 seconds. The difference is 6 - 2 = 4 m/s (This is the speed of walking of the person)
9-3=6 m/s

 

[Student100]

Well, I get 2 m/s and 3 m/s for 30 seconds only. (This is the speed of the elevator, so the elevator is constant) I get 6 m/s and 9 m/s for 10 seconds. The difference is 6 - 2 = 4 m/s (This is the speed of walking of the person)
9-6=6 m/s

Okay, now how do you find the time from the 4m/s or 6m/s?

 

[astrololo]

Okay, now how do you find the time from the 4m/s or 6m/s?

Oh so the time is 15 seconds. So from my understanding, whether he walks at 4 m/s or 6 m/s, it takes him 15 seconds to climb the stairs of the non working elevator ?

 

[Student100]

Oh so the time is 15 seconds. So from my understanding, whether he walks at 4 m/s or 6 m/s, it takes him 15 seconds to climb the stairs of the non working elevator ?

Yep, no matter the distance, the time is invariable that he takes to walk up the broken escalator. Some distances produce silly physical results (walking at incredible velocities), but that's the basis of this problem.

Now you can answer the next problem relatively easily.

 

[astrololo]

Yep, no matter the distance, the time is invariable. Some distances produce silly physical results, but that's the basis of this problem.

Now you can answer the next problem relatively easily.

So, the answer to the next problem would be no I can't guess the "Real" distance because there's no information indicating an exact distance for the elevator ? (We assumed distances to solve the problem)

 

[Student100]

So, the answer to the next problem would be no I can't guess the "Real" distance because there's no information indicating an exact distance for the elevator ? (We assumed distances to solve the problem)

Basically, it's impossible to know the distance from the information given. A longer distance implies that the escalator and his walking is at faster velocity, since the time doesn't change.

Does this all make sense to you? Do you see the importance of not trying to plug things into equations immediately and looking at the physical aspect of the problem?

 

[astrololo]

Basically, it's impossible to know the distance from the information given. A longer distance implies that the escalator and his walking is at faster velocity, since the time doesn't change.

Yes, that's right. There's no information for the distance. And also, liek you said, to keep the time the same, we need to make the datas grow. Thank you very much for your time and patience !

 

[astrololo]

Basically, it's impossible to know the distance from the information given. A longer distance implies that the escalator and his walking is at faster velocity, since the time doesn't change.

Does this all make sense to you? Do you see the importance of not trying to plug things into equations immediately and looking at the physical aspect of the problem?

Yes. What I think I should have done is trying some random distances for the escalator. I was sure that there was no way of guessing the distance from just the time, so I should have been intelligent in trying different distances to see where it would lead. After that, I would have understood more the situation.

 

[Student100]

Yes. What I think I should have done is trying some random distances for the escalator. I was sure that there was no way of guessing the distance from just the time, so I should have been intelligent in trying different distances to see where it would lead. After that, I would have understood more the situation.

It's always difficult to learn the heuristics, but once you do you'll find these kinds of problems a lot easier. Good job.

 

[astrololo]

It's always difficult to learn the heuristics, but once you do you'll find these kinds of problems a lot easier. Good job.

One last point. My main problem is that I'm learning in a school context. So if I see that I take too much time I go ask the questions directly. I know that I could stare at the problem for days trying different approaches and methods for solving it, but I can't do that because you know, I have a sandglass that it's in my head lol

 

[Student100]

One last point. My main problem is that I'm learning in a school context. So if I see that I take too much time I go ask the questions directly. I know that I could stare at the problem for days trying different approaches and methods for solving it, but I can't do that because you know, I have a sandglass that it's in my head lol

That's fine as long as the people you're asking are guiding you to the solution, not just giving it to you. Keep posting here, people will more or less help you arrive at the solution yourself. The whole point of these problems is to help you develop techniques and problem solving ideas so that when you become a researcher or engineer and encounter a new problem you have tools to search for a solution.

I always found it best to skip over problems I didn't know and knock out those that I did. That way I'm left with a few problems from the problem set and days to think about them. So if you're stuck, just finish what you can and those problems you have trouble with really think about them in detail.

I'm sure you'll do fine, good luck!

 

[insightful]

Just so the OP sees what the "classical" solution looks like:
Let L = length of escalator
Let Ve = velocity of escalator
Let Vw = velocity of walking
Let Vew = velocity of escalator and walking
Let T = unknown time of only walking up broken escalator
velocity = distance/time
Ve = L/30
Vew = L/10
Vw = L/T
Vew = Ve + Vw
L/10 = L/30 + L/T
T = 15 seconds
and clearly the answer is independent of L.